Contents
Images
Upload your image
DSS Images Other Images
Related articles
Investigating the Cosmic-Ray Ionization Rate in the Galactic Diffuse Interstellar Medium through Observations of H+ 3
Observations of H+ 3 in the Galactic diffuseinterstellar medium have led to various surprising results, includingthe conclusion that the cosmic-ray ionization rate (?2)is about one order of magnitude larger than previously thought. Thepresent survey expands the sample of diffuse cloud sight lines withH+ 3 observations to 50, with detections in 21 ofthose. Ionization rates inferred from these observations are in therange (1.7 ± 1.3) × 10-16s-1 < ?2 < (10.6 ± 8.2)× 10-16 s-1 with a mean value of?2 = (3.5+5.3 -3.0) ×10-16 s-1. Upper limits (3?)derived from non-detections of H+ 3 are as low as?2 < 0.4 × 10-16s-1. These low upper limits, in combination with thewide range of inferred cosmic-ray ionization rates, indicate variationsin ?2 between different diffuse cloud sight lines. Astudy of ?2 versus N H (total hydrogen columndensity) shows that the two parameters are not correlated for diffusemolecular cloud sight lines, but that the ionization rate decreases whenN H increases to values typical of dense molecular clouds.Both the difference in ionization rates between diffuse and dense cloudsand the variation of ?2 among diffuse cloud sight linesare likely the result of particle propagation effects. The lowerionization rate in dense clouds is due to the inability of low-energy(few MeV) protons to penetrate such regions, while the ionization ratein diffuse clouds is controlled by the proximity of the observed cloudto a site of particle acceleration.
| The VLT-FLAMES survey of massive stars: NGC 346-013 as a test case for massive close binary evolution
Context. NGC 346-013 is a peculiar double-lined eclipsing binary in theSmall Magellanic Cloud (SMC) discovered by the VLT-FLAMES survey ofmassive stars. Aims: We use spectroscopic and photometricobservations to investigate the physical properties and evolutionaryhistory of NGC 346-013. Methods: Spectra obtained with VLT/FLAMESare used to construct a radial velocity curve for NGC 346-013 and tocharacterise the early B-type secondary. Photometry obtained with theFaulkes Telescope South is then used to derive orbital parameters, whilespectra of the secondary are compared with synthetic spectra from TLUSTYmodel atmospheres. Results: The orbital period is found to be4.20381(12) days, with masses of 19.1 ± 1.0 and 11.9 ± 0.6M&sun;. The primary is a rapidly rotating (vrot =320 ± 30 km s-1) late-O dwarf while the secondary, anearly-B giant, displays near-synchronous rotation and has filled itsRoche lobe, implying that it was originally the more massive componentwith recent mass transfer "spinning up" the primary to near-criticalrotation. Comparison with synthetic spectra finds temperatures of 34.5kK and 24.5 kK for the primary and secondary respectively, with thenitrogen abundance of the secondary enhanced compared to baseline valuesfor the SMC, consistent with the predictions of models of interactingbinaries. Conclusions: NGC 346-013 likely evolved vianon-conservative mass transfer in a system with initial masses ~22+15M&sun;, with the well-constrained orbital solution andatmospheric parameters making it an excellent candidate for tailoredmodelling with binary evolution codes. This system will form acornerstone in constraining the physics of thermal timescale masstransfer, and the associated mass transfer efficiency, in massive closebinary systems.Based on observations at the European Southern Observatory, Paranal,Chile in programmes 171.D-0237 and 081.D-0364.
| A multi-epoch XMM-Newton campaign on the core of the massive Cygnus OB2 association
Context. Cyg OB2 is one of the most massive associations of O-type starsin our Galaxy. Despite the large interstellar reddening towards Cyg OB2,many studies, spanning a wide range of wavelengths, have been conductedto more clearly understand this association. X-ray observations providea powerful tool to overcome the effect of interstellar absorption andstudy the most energetic processes associated with the stars in Cyg OB2. Aims: We analyse XMM-Newton data to investigate the X-ray and UVproperties of massive O-type stars as well as low-mass pre-main sequencestars in Cyg OB2. Methods: We obtained six XMM-Newtonobservations of the core of Cyg OB2. In our analysis, we pay particularattention to the variability of the X-ray bright OB stars, especiallythe luminous blue variable candidate Cyg OB2 #12. Results: Wefind that X-ray variability is quite common among the stars in Cyg OB2.Whilst short-term variations are restricted mostly to low-mass pre-mainsequence stars, one third of the OB stars display long-term variations.The X-ray flux of Cyg OB2 #12 varies by 37%, over timescales from daysto years, whilst its mean log LX/Lbol amounts to -6.10. Conclusions: These properties suggest that Cyg OB2 #12 iseither an interacting-wind system or displays a magnetically confinedwind. Two other X-ray bright O-type stars (MT91 516 and CPR2002 A11)display variations that suggest they are interacting wind binarysystems.Based on observations collected with XMM-Newton, an ESA Science Missionwith instruments and contributions directly funded by ESA Member Statesand the USA (NASA).Full Table 2 and Table 4 are only available inelectronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr(130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/536/A31
| Observation of the Cygnus OB2 Association with Suzaku and Long Term Light Curves
We obtained a Suzaku 41-ks observation of the four brightest early typestars in the Cygnus OB2 association (Cyg OB2 Nos. 5, 8a, 9, and 12). Thespectra of two of the stars, Nos. 9 and 12, could be fitted by atwo-temperature thin-thermal collisional-ionization equilibrium model,while those of Nos. 5 and 8a required a three-temperature model. StarNo. 8a showed a time variation both above 3 keV and below 1.5 keV. Thetime scale of the variation of the soft X-rays is ˜ 8 ×104 s, while that above 3 keV is ˜ 2 ×104 s. We found that the variation above 3 keV can beexplained by temperature changes in the hottest plasma of thethree-temperature model. This result restricts the size and density ofthe hottest component of the three-temperature model. We also producedlong-term light curves of the four stars for over 30 yr. We confirmedthe orbital modulation in the intensity of No. 8a, and discovered apossible orbital modulation of 2.35 yr in the light curve of No. 9.
| The Non-thermal, Time-variable Radio Emission from Cyg OB2 #5: A Wind-collision Region
The radio emission from the well-studied massive stellar system Cyg OB2#5 is known to fluctuate with a period of 6.7 years between a low-fluxstate, when the emission is entirely of free-free origin, and ahigh-flux state, when an additional non-thermal component (of hithertounknown nature) appears. In this paper, we demonstrate that the radioflux of that non-thermal component is steady on timescales of hours andthat its morphology is arc-like. This shows that the non-thermalemission results from the collision between the strong wind driven bythe known contact binary in the system and that of an unseen companionon a somewhat eccentric orbit with a 6.7 year period and a 5-10 massemimajor axis. Together with the previously reported wind-collisionregion located about 0farcs8 to the northeast of the contact binary, sofar Cyg OB2 #5 appears to be the only multiple system known to harbortwo radio-imaged wind-collision regions.
| Random forest automated supervised classification of Hipparcos periodic variable stars
We present an evaluation of the performance of an automatedclassification of the Hipparcos periodic variable stars into 26 types.The sub-sample with the most reliable variability types available in theliterature is used to train supervised algorithms to characterize thetype dependencies on a number of attributes. The most useful attributesevaluated with the random forest methodology include, in decreasingorder of importance, the period, the amplitude, the V-I colour index,the absolute magnitude, the residual around the folded light-curvemodel, the magnitude distribution skewness and the amplitude of thesecond harmonic of the Fourier series model relative to that of thefundamental frequency. Random forests and a multi-stage scheme involvingBayesian network and Gaussian mixture methods lead to statisticallyequivalent results. In standard 10-fold cross-validation (CV)experiments, the rate of correct classification is between 90 and 100per cent, depending on the variability type. The main mis-classificationcases, up to a rate of about 10 per cent, arise due to confusion betweenSPB and ACV blue variables and between eclipsing binaries, ellipsoidalvariables and other variability types. Our training set and thepredicted types for the other Hipparcos periodic stars are availableonline.
| An R- and I-band Photometric Variability Survey of the Cygnus OB2 Association
We present a catalog of photometrically variable stars discovered withintwo 21farcm3 × 21farcm3 fields centered on the Cygnus OB2association (Cyg OB2). There have hitherto been no deep opticalvariability studies of Cyg OB2, despite it being replete with early-typemassive stars, likely due to the high and variable extinction (up toAV ? 20) that permeates much of the region. Here, weprovide results of the first variability study with this combination ofspatial coverage (~0.5 deg) and photometric depth (R ~ 21 mag). We find121 stars to be variable in both R and I bands, 115 of them newlydiscovered. Of the 121 variables, we identify 27 eclipsing binaries andeclipsing-binary candidates, 52 pulsating variables, and 20 potentialHerbig Ae/Be stars. Confirming both the status and the clustermembership of the Herbig Ae/Be stars would address the uncertaintyregarding the age and star formation history of Cyg OB2. We match ourcatalog to known variables and binaries in the region, Two Micron AllSky Survey near-IR data, and Chandra X-ray observations to findcounterparts to new variables in other wavelengths.Based on observations obtained using the 1.3 m McGraw-Hill telescope atthe MDM Observatory.
| The VLT-FLAMES Tarantula Survey. II. R139 revealed as a massive binary system
We report the discovery that R139 in 30 Doradus is a massivespectroscopic binary system. Multi-epoch optical spectroscopy of R139was obtained as part of the VLT-FLAMES Tarantula Survey, revealing adouble-lined system. The two components are of similar spectral types;the primary exhibits strong C III ?4650 emission and isclassified as an O6.5 Iafc supergiant, while the secondary is an O6 Iafsupergiant. The radial-velocity variations indicate a highly eccentricorbit with a period of 153.9 days. Photometry obtained with the FaulkesTelescope South shows no evidence for significant variability within an18 month period. The orbital solution yields lower mass limits for thecomponents of M1sin3i = 78 ± 8M&sun; and M2sin3i = 66 ± 7M&sun;. As R139 appears to be the most massive binary systemknown to contain two evolved Of supergiants, it will provide anexcellent test for atmospheric and evolutionary models.
| The Galactic O-Star Spectroscopic Survey. I. Classification System and Bright Northern Stars in the Blue-violet at R ~ 2500
We present the first installment of a massive spectroscopic survey ofGalactic O stars, based on new, high signal-to-noise ratio, R ~ 2500digital observations from both hemispheres selected from the GalacticO-Star Catalog of Maíz Apellániz et al. and Sota et al.The spectral classification system is rediscussed and a new atlas ispresented, which supersedes previous versions. Extensive sequences ofexceptional objects are given, including types Ofc, ON/OC, Onfp, Of?p,Oe, and double-lined spectroscopic binaries. The remaining normalspectra bring this first sample to 184 stars, which is close to completeto B = 8 and north of ? = -20° and includes all of thenorthern objects in Maíz Apellániz et al. that are stillclassified as O stars. The systematic and random accuracies of theseclassifications are substantially higher than previously attainable,because of the quality, quantity, and homogeneity of the data andanalysis procedures. These results will enhance subsequentinvestigations in Galactic astronomy and stellar astrophysics. In thefuture, we will publish the rest of the survey, beginning with a secondpaper that will include most of the southern stars in MaízApellániz et al.The spectroscopic data in this article were gathered with threefacilities: the 1.5 m telescope at the Observatorio de Sierra Nevada(OSN), the 3.5 m telescope at Calar Alto Observatory (CAHA), and the duPont 2.5 m telescope at Las Campanas Observatory (LCO). Some of thesupporting imaging data were obtained with the 2.2 m telescope at CAHAand the NASA/ESA Hubble Space Telescope (HST). The rest were retrievedfrom the DSS2 and Two Micron All Sky Survey (2MASS) surveys. The HSTdata were obtained at the Space Telescope Science Institute, which isoperated by the Association of Universities for Research in Astronomy,Inc., under NASA contract NAS 5-26555.
| Polycyclic Aromatic Hydrocarbons and the Diffuse Interstellar Bands: A Survey
We discuss the proposal of relating the origin of some of the diffuseinterstellar absorption bands (DIBs) to neutral polycyclic aromatichydrocarbons (PAHs) present in translucent interstellar clouds. Anassessment of ionized PAHs will be examined in a future report. Thespectra of several cold, isolated gas-phase PAHs have been measured inthe laboratory under experimental conditions that mimic the interstellarconditions and are compared with an extensive set of astronomicalspectra of reddened, early-type stars. This comparisonprovides—for the first time—accurate upper limits for theabundances of specific PAH molecules along specific lines of sight,something not attainable from IR observations alone. The comparison ofthese unique laboratory data with high-resolution, high signal-to-noiseratio spectra leads to two major findings: (1) a finding specific to theindividual molecules that were probed in this study and, which leads tothe clear and unambiguous conclusion that the abundance of thesespecific neutral PAHs must be very low in the individual translucentinterstellar clouds that were probed in this survey (PAH features remainbelow the level of detection) and, (2) a general finding that neutralPAHs exhibit intrinsic band profiles that are similar to the profile ofthe narrow DIBs indicating that the carriers of the narrow DIBs musthave close molecular structure and characteristics. This study is thefirst quantitative survey of neutral PAHs in the optical range and itopens the way for unambiguous quantitative searches of PAHs in a varietyof interstellar and circumstellar environments.
| Studies of Diffuse Interstellar Bands V. Pairwise Correlations of Eight Strong DIBs and Neutral Hydrogen, Molecular Hydrogen, and Color Excess
We establish correlations between equivalent widths of eight diffuseinterstellar bands (DIBs), and examine their correlations with atomichydrogen, molecular hydrogen, and E B-V . The DIBs arecentered at ?? 5780.5, 6204.5, 6283.8, 6196.0, 6613.6,5705.1, 5797.1, and 5487.7, in decreasing order of Pearson's correlationcoefficient with N(H) (here defined as the column density of neutralhydrogen), ranging from 0.96 to 0.82. We find the equivalent width (EW)of ?5780.5 is better correlated with column densities of H thanwith E B-V or H2, confirming earlier resultsbased on smaller data sets. We show that the same is true for six of theseven other DIBs presented here. Despite this similarity, the eightstrong DIBs chosen are not correlated well enough with each other tosuggest they come from the same carrier. We further conclude that theseeight DIBs are more likely to be associated with H than withH2, and hence are not preferentially located in the densest,most UV shielded parts of interstellar clouds. We suggest that theyarise from different molecules found in diffuse H regions with verylittle H2 (molecular fraction f < 0.01). Of the 133 starswith available data in our study, there are three with significantlyweaker ?5780.5 than our mean H-?5780.5 relationship, allof which are in regions of high radiation fields, as previously noted byHerbig. The correlations will be useful in deriving interstellarparameters when direct methods are not available. For instance, withcare, the value of N(H) can be derived from W?(5780.5).
| The masses, and the mass discrepancy of O-type stars
Context. The “mass discrepancy” in massive O starsrepresents a long-standing problem in stellar astrophysics withfar-reaching implications for the chemical and dynamical feedback ingalaxies. Aims: Our goal is to investigate this mass discrepancyby comparing state-of-the-art model masses with model-independent massesdetermined from eclipsing binaries. Methods: Using stellarevolution models and a recent calibration of stellar parameters forO-star spectral sub-classes, we present a convenient way to convertobserved solar metallicity O star spectral types into model masses,which we subsequently compare to our dynamical mass compilation. We alsoderive similar conversions for Large and Small Magellanic Cloudmetallicities. Results: We obtain a good agreement between modeland dynamical masses, suggesting the long-standing problem of asystematic mass discrepancy problem may have been solved. We alsoprovide error ranges for the model masses, as well as minimal andmaximal age estimates for when the model stars are in a given spectraltype box.Appendices and Table 9 are only available in electronic form at http://www.aanda.org
| On the Size of the Non-Thermal Component in the Radio Emission from Cyg OB2 #5
Cyg OB2 #5 is a contact binary system with variable radio continuumemission. This emission has a low-flux state where it is dominated bythermal emission from the ionized stellar wind and a high-flux statewhere an additional non-thermal component appears. The variations arenow known to have a period of 6.7±0.2 yr. The non-thermalcomponent has been attributed to different agents: an expanding envelopeejected periodically from the binary, emission from a wind-collisionregion, or a star with non-thermal emission in an eccentric orbit aroundthe binary. The determination of the angular size of the non-thermalcomponent is crucial to discriminate between these alternatives. Wepresent the analysis of VLA archive observations made at 8.46 GHz in1994 (low state) and 1996 (high state), that allow us to subtract theeffect of the persistent thermal emission and to estimate an angularsize of ? 0.''02 for the non-thermal component. This compact sizefavors the explanation in terms of a star with non-thermal emission orof a wind-collision region.
| A new Bok globule towards Cygnus OB2 No. 12
The NH3(1, 1) line emission was detected at -120 arcsec, + 60arcsec offset from the star Cyg OB2 No. 12. The region had beeninitially mapped in 13CO(1 -0) and was called clump B. Thesubsequent detection of the CS(2 -1) line by the same authors led themto suggest that the clump could be an object similar to a Bok globule.The present detection and analysis confirm what was proposed before,that is clump B is a newly discovered starless Bok globule.
| A VLT/FLAMES survey for massive binaries in Westerlund 1. II. Dynamical constraints on magnetar progenitor masses from the eclipsing binary W13
Context. Westerlund 1 is a young, massive Galactic starburst clusterthat contains a rich coeval population of Wolf-Rayet stars, hot- andcool-phase transitional supergiants, and a magnetar. Aims: We usespectroscopic and photometric observations of the eclipsing double-linedbinary W13 to derive dynamical masses for the two components, in orderto determine limits for the progenitor masses of the magnetar CXOUJ164710.2-455216 and the population of evolved stars in Wd1. Methods: We use eleven epochs of high-resolution VLT/FLAMES spectroscopyto construct a radial velocity curve for W13. R-band photometry is usedto constrain the inclination of the system. Results: W13 has anorbital period of 9.2709 ± 0.0015 days and near-contactconfiguration. The shallow photometric eclipse rules out an inclinationgreater than 65°, leading to lower limits for the masses of theemission-line optical primary and supergiant optical secondary of 21.4± 2.6 M_&sun; and 32.8 ± 4.0 M_&sun; respectively, risingto 23.2-3.0+3.3 M_&sun; and35.4-4.6+5.0 M_&sun; for our best-fit inclination62-4+3 degrees. Comparison with theoretical modelsof Wolf-Rayet binary evolution suggest the emission-line object had aninitial mass in excess of ~35 M_&sun;, with the most likely modelfeaturing highly non-conservative late-Case A/Case B mass transfer andan initial mass in excess of 40 M_&sun;. Conclusions: This resultconfirms the high progenitor mass of the magnetar CXOU J164710.2-455216inferred from its membership in Wd1, and represents the first dynamicalconstraint on the progenitor mass of any magnetar. The red supergiantsin Wd1 must have similar progenitor masses to W13 and are thereforeamongst the most massive stars to undergo a red supergiant phase,representing a challenge for population models that suggest stars inthis mass range end their redwards evolution as yellow hypergiants.This work is based on observations collected at the European SouthernObservatory under programme IDs ESO 81.D-0324 and 383.D-0633.
| Non-thermal radio emission from O-type stars. IV. Cygnus OB2 No. 8A
Context. Several early-type colliding-wind binaries are known to emitsynchrotron radiation due to relativistic electrons, which are mostprobably accelerated by the Fermi mechanism. By studying such systems wecan learn more about this mechanism, which is also relevant in otherastrophysical contexts. Colliding-wind binaries are furthermoreimportant for binary frequency determination in clusters and forunderstanding clumping and porosity in stellar winds. Aims: Westudy the non-thermal radio emission of the binary Cyg OB2 No. 8A, tosee if it is variable and if that variability is locked to the orbitalphase. We investigate if the synchrotron emission generated in thecolliding-wind region of this binary can explain the observations and weverify that our proposed model is compatible with the X-ray data. Methods: We use both new and archive radio data from the Very LargeArray (VLA) to construct a light curve as a function of orbital phase.We also present new X-ray data that allow us to improve the X-ray lightcurve. We develop a numerical model for the colliding-wind region andthe synchrotron emission it generates. The model also includes free-freeabsorption and emission due to the stellar winds of both stars. In thisway we construct artificial radio light curves and compare them with theobserved one. Results: The observed radio fluxes showphase-locked variability. Our model can explain this variability becausethe synchrotron emitting region is not completely hidden by thefree-free absorption. In order to obtain a better agreement for thephases of minimum and maximum flux we need to use stellar windparameters for the binary components which are somewhat different fromtypical values for single stars. We verify that the change in stellarparameters does not influence the interpretation of the X-ray lightcurve. Our model has trouble explaining the observed radio spectralindex. This could indicate the presence of clumping or porosity in thestellar wind, which - through its influence on both the Razin effect andthe free-free absorption - can considerably influence the spectralindex. Non-thermal radio emitters could therefore open a valuablepathway to investigate the difficult issue of clumping in stellar winds.Partly based on observations with XMM-Newton, an ESA Science Missionwith instruments and contributions directly funded by ESA Member Statesand the USA (NASA).Appendices are only available in electronic form athttp://www.aanda.org
| The most plausible explanation of the cyclic period changes in close binaries: the case of the RS CVn-type binary WW Dra
Cyclic period changes are a fairly common phenomenon in close binarysystems and are usually explained as being caused either by the magneticactivity of one or both components or by the light travel time effect(LTTE) of a third body. We searched the orbital period changes in 182EA-type (including the 101 Algol systems used by Hall), 43 EB-type and53 EW-type binaries with known mass ratio and spectral type of thesecondary component. We reproduced and improved the diagram in Hallaccording to the new collected data. Our plots do not support theconclusion derived by Hall that cyclic period changes are restricted tobinaries having a secondary component with spectral type later than F5.The presence of period changes among systems with a secondary componentof early type indicates that magnetic activity is one, but not the only,cause of the period variation. It is discovered that cyclic periodchanges, probably resulting from the presence of a third body, are morefrequent in EW-type binaries among close systems. Therefore, the mostplausible explanation of the cyclic period changes is the LTTE throughthe presence of a third body. Using the century-long historical recordof the times of light minimum, we analysed the cyclic period change inthe Algol binary WW Dra. It is found that the orbital period of thebinary shows a ~112.2-yr cyclic variation with an amplitude of ~0.1977d.The cyclic oscillation can be attributed to the LTTE by means of a thirdbody with a mass no less than 6.43Msolar. However, nospectral lines of the third body were discovered, indicating that it maybe a candidate black hole. The third body is orbiting the binary at adistance closer than 14.4 au and may play an important role in theevolution of this system.
| High-resolution imaging of Galactic massive stars with AstraLux. I. 138 fields with ? > - 25°
Context. Massive stars have high-multiplicity fractions, and many ofthem have still undetected components, thus hampering the study of theirproperties. Aims: I study a sample of massive stars with highangular resolution to better characterize their multiplicity. Methods: I observed 138 fields that include at least one massive starwith AstraLux, a lucky imaging camera at the 2.2 m Calar Alto telescope.I also used observations of 3 of those fields with ACS/HRC on HST toobtain complementary information and to calibrate the AstraLux data. Theresults were compared with existing information from the WashingtonDouble Star Catalog, Tycho-2, 2MASS, and other literature results. Results: I discover 16 new optical companions of massive stars, themajority of which are likely to be physically bound to their primaries.I also improve the accuracy for the separation and magnitude differenceof many previously known systems. In a few cases the orbital motion isdetected when comparing the new data with existing ones and constraintson the orbits are provided. Conclusions: The analysis indicatethat the majority of the AstraLux detections are bound pairs. For arange of separations of 0.1 arcsec-14'' and magnitude differences lowerthan 8, I find that the multiplicity fraction for massive stars is closeto 50%. When objects outside those ranges are included, the multiplicityfraction should be considerably higher.Table 3 is also available in electronic form at the CDS via anonymousftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/518/A1
| The Chandra Variable Guide Star Catalog
Variable stars have been identified among the optical-wavelength lightcurves of guide stars used for pointing control of the Chandra X-rayObservatory. We present a catalog of these variable stars along withtheir light curves and ancillary data. Variability was detected to alower limit of 0.02 mag amplitude in the 4000-10000 Å range usingthe photometrically stable Aspect Camera on board the Chandraspacecraft. The Chandra Variable Guide Star Catalog (VGUIDE) contains827 stars, of which 586 are classified as definitely variable and 241are identified as possibly variable. Of the 586 definite variable stars,we believe 319 are new variable star identifications. Types of variablesin the catalog include eclipsing binaries, pulsating stars, and rotatingstars. The variability was detected during the course of normalverification of each Chandra pointing and results from analysis of over75,000 guide star light curves from the Chandra mission. The VGUIDEcatalog represents data from only about 9 years of the Chandra mission.Future releases of VGUIDE will include newly identified variable guidestars as the mission proceeds. An important advantage of the use ofspace data to identify and analyze variable stars is the relatively longobservations that are available. The Chandra orbit allows forobservations up to 2 days in length. Also, guide stars were often usedmultiple times for Chandra observations, so many of the stars in theVGUIDE catalog have multiple light curves available from various timesin the mission. The catalog is presented as both online data associatedwith this paper and as a public Web interface. Light curves with data atthe instrumental time resolution of about 2 s, overplotted with the databinned at 1 ks, can be viewed on the public Web interface and downloadedfor further analysis. VGUIDE is a unique project using data collectedduring the mission that would otherwise be ignored. The stars availablefor use as Chandra guide stars are generally 6-11 mag and are commonlyspectral types A and later. Due to the selection of guide stars entirelyfor positional convenience, this catalog avoids the possible bias ofsearching for variability in objects where it is to be expected.Statistics of variability compared to spectral type indicate theexpected dominance of A-F stars as pulsators. Eclipsing binaries areconsistently 20%-30% of the detected variables across all spectraltypes.
| Masses and luminosities of O- and B-type stars and red supergiants
Massive stars are of interest as progenitors of supernovae, i.e.neutron stars and black holes, which can be sources of gravitationalwaves. Recent population synthesis models can predict neutron star andgravitational wave observations but deal with a fixed supernova rate oran assumed initial mass function for the population of massive stars. Here we investigate those massive stars, which are supernovaprogenitors, i.e. with O- and early B-type stars, and also allsupergiants within 3 kpc. We restrict our sample to those massive starsdetected both in 2MASS and observed by Hipparcos, i.e. only those starswith parallax and precise photometry. To determine the luminositieswe calculated the extinctions from published multi-colour photometry,spectral types, luminosity class, all corrected for multiplicity andrecently revised Hipparcos distances. We use luminosities andtemperatures to estimate the masses and ages of these stars usingdifferent models from different authors. Having estimated theluminosities of all our stars within 3 kpc, in particular for all O- andearly B-type stars, we have determined the median and mean luminositiesfor all spectral types for luminosity classes I, III, and V. Ourluminosity values for supergiants deviate from earlier results: Previouswork generally overestimates distances and luminosities compared to ourdata, this is likely due to Hipparcos parallaxes (generally moreaccurate and larger than previous ground-based data) and the fact thatmany massive stars have recently been resolved into multiples of lowermasses and luminosities. From luminosities and effective temperatureswe derived masses and ages using mass tracks and isochrones fromdifferent authors. From masses and ages we estimated lifetimes andderived a lower limit for the supernova rate of ?20 events/Myraveraged over the next 10 Myr within 600 pc from the sun. These data arethen used to search for areas in the sky with higher likelihood for asupernova or gravitational wave event (like OB associations).
| HD 93129A at different radio scales
Recent observations toward the O2 If* star HD 93129A have revealedimportant non-thermal radio emission, and that it has an early Ocompanion 140 AU away if at 2.5 kpc (FGS-HST data). These facts stronglysupport the possibility of a massive stars wind-collision region as theacceleration site of the synchrotron-emitting relativistic electrons. Wehave carried out LBA VLBI observations at 2.37 GHz to resolve thenon-thermal source, and search for a bow-shock shape, typical of awind-collision region. Preliminary results are presented here.
| Modeling the Radio Emission from Cyg OB2 No. 5: A Quadruple System?
Fifty observations at frequencies between 1.4 GHz and 43 GHz of the 6.6day O6.5-7+O5.5-6 binary Cyg OB2 No. 5 using the Very Large Array over20 years are re-examined. The aim is to determine the location andcharacter of the previously detected variable radio emission. The radioemission from the system consists of a primary component that isassociated with the binary, and a non-thermal source (NE), 0farcs8 tothe NE of the binary that has been ascribed to a wind-collision region(WCR) between the stellar winds of the binary and that of a B-type star(Star D) to the NE. Previous studies have not accounted for thepotential contribution of NE to the total radio emission, mostespecially in observations where the primary and NE sources are notresolved as separate sources. NE shows no evidence of variation in 23epochs where it is resolved separately from the primary radio component,demonstrating that the variable emission arises in the primarycomponent. Since NE is non-variable, the radio flux from the primary cannow be well determined for the first time, most especially inobservations that do not resolve both the primary and NE components. Thevariable radio emission from the primary component has a period of 6.7± 0.3 years which is described by a simple model of a non-thermalsource orbiting within the stellar wind envelope of the binary. Such amodel implies the presence of a third, unresolved stellar companion(Star C) orbiting the 6.6 day binary with a period of 6.7 years andindependent of Star D to the NE. The variable non-thermal emissionarises from either a WCR between Star C and the binary system, orpossibly from Star C directly. The model gives a mass-loss rate of 3.4× 10-5 M sun yr-1 for Cyg OB2 No.5, unusually high for an Of supergiant and comparable to that of WRstars, and consistent with an unusually strong He I 1.083 µmemission line, also redolent of WR stars. An examination of radialvelocity observations available from the literature suggests reflexmotion of the binary due to Star C, for which a mass of 23+22-14 M sun is deduced. The natures of NE and Star Dare also examined. If NE is a WCR, as suggested by other authors, thenthe required mass-loss rate is an order of magnitude higher thanexpected for an early B-type dwarf, and only just consistent with asupergiant. This raises the question of NE as a WCR, but its non-thermalluminosity is consistent with a WCR and a comparison of reddeningbetween Cyg OB2 No. 5 and Star D do not rule out an association,implying Cyg OB2 No. 5 is a quadruple system. Pursuing alternativemodels for NE, such as an unassociated background source, would requirevery challenging observations.
| The eclipsing, double-lined, Of supergiant binary Cygnus OB2-B17
Context. Massive, eclipsing, double-lined, spectroscopic binaries arenot common but are necessary to understand the evolution of massivestars as they are the only direct way to determine stellar masses. Theyare also the progenitors of energetic phenomena such as X-ray binariesand ?-ray bursts. Aims: We present a photometric andspectroscopic analysis of the candidate binary system Cyg OB2-B17 toshow that it is indeed a massive evolved binary. Methods: Weutilise V band and white-light photometry to obtain a light curve andperiod of the system, and spectra at different resolutions to calculatepreliminary orbital parameters and spectral classes for the components. Results: Our results suggest that B17 is an eclipsing,double-lined, spectroscopic binary with a period of 4.0217±0.0004days, with two massive evolved components with preliminaryclassifications of O7 and O9 supergiants. The radial velocity and lightcurves are consistent with a massive binary containing components withsimilar luminosities, and in turn with the preliminary spectral typesand age of the association.
| Studies of the Diffuse Interstellar Bands. IV. The Nearly Perfect Correlation Between ??6196.0 and 6613.6
In a sample of 114 diffuse cloud sightlines spanning a wide range ofinterstellar environments, we find the equivalent widths of the diffuseinterstellar bands (DIBs) ?6196.0 and ?6613.6 to beextremely well correlated, with a correlation coefficient of 0.986. Amaximum likelihood functional relationship analysis shows that theobservations are consistent with a perfect correlation if theobservational errors, which are dominated by continuum placement andother systematics such as interfering lines, have been underestimated bya factor of 2. The quality of this correlation far exceeds otherpreviously studied correlations, such as that between the ?5780.5DIB and either the color excess or the atomic hydrogen column density.The unusually tight correlation between these two DIBs would seem tosuggest that they might represent the first pair of DIBs known to be dueto the same molecular carrier. However, further theoretical work will berequired to determine whether the different linewidths and band shapesof these two DIBs can be consistent with a common carrier. If the twoDIBs do not in fact share the same molecular carrier, their two carriersmust be chemically very closely related.
| Gamma-ray emission from star-forming regions .
Star-forming regions have been tentatively associated with gamma-raysources since the early days of the COS B satellite. After the manysources detected by the Compton Gamma-Ray Observatory, the statisticalevidence for such an association has became overwhelming. Recent resultsfrom imaging atmospheric Cherenkov telescopes indicate that somehigh-energy sources are produced in regions of active star formationlike Cygnus OB2 and Westerlund 2. In this paper I will review what kindof stellar objects can produce gamma-ray emission in star-formingregions and I will suggest that the formation process of massive starscould in principle result in the production of observable gamma rayemission. Gamma-ray astronomy, will be argued, is mature enough as tocontribute to the study of the birth of massive stars.
| A systematic study of variability among OB-stars based on HIPPARCOS photometry
Context: Variability is a key factor for understanding the nature of themost massive stars, the OB stars. Such stars lie closest to the unstableupper limit of star formation. Aims: In terms of statistics, thedata from the HIPPARCOS satellite are unique because of time coverageand uniformity. They are ideal to study variability in this large,uniform sample of OB stars. Methods: We used statisticaltechniques to determine an independant threshold of variabilitycorresponding to our sample of OB stars, and then applied an automaticalgorithm to search for periods in the data of stars that are locatedabove this threshold. We separated the sample stars into 4 maincategories of variability: 3 intrinsic and 1 extrinsic. The intrinsiccategories are: OB main sequence stars (~2/3 of the sample), OBe stars(~10%) and OB Supergiant stars (~1/4).The extrinsic category refers toeclipsing binaries. Results: We classified about 30% of the wholesample as variable, although the fraction depends on magnitude level dueto instrumental limitations. OBe stars tend to be much more variable(≈80%) than the average sample star, while OBMS stars are belowaverage and OBSG stars are average. Types of variables include αCyg, β Cep, slowly pulsating stars and other types from the generalcatalog of variable stars. As for eclipsing binaries, there arerelatively more contact than detached systems among the OBMS and OBestars, and about equal numbers among OBSG stars.
| Hot stars observed by XMM-Newton. I. The catalog and the properties of OB stars
Aims: Following the advent of increasingly sensitive X-rayobservatories, deep observations of early-type stars became possible.However, the results for only a few objects or clusters have until nowbeen reported and there has been no large survey comparable to thatbased upon the ROSAT All-Sky Survey (RASS). Methods: A limitedsurvey of X-ray sources, consisting of all public XMM-Newtonobservations (2XMMi) and slew survey data (XMMSL1), is now available.The X-ray counterparts to hot, massive stars have been searched for inthese catalogs. Results: About 300 OB stars were detected withXMM-Newton. Half of them were bright enough for a spectral analysis tobe possible, and we make available the detailed spectral properties thatwere derived. The X-ray spectra of O stars are represented well by low(<1 keV) temperature components and seem to indicate that anabsorption column is present in addition to the interstellarcontribution. The X-ray fluxes are well correlated with the bolometricfluxes, with a scatter comparable to that of the RASS studies and thuslarger than found previously with XMM-Newton for some individualclusters. These results contrast with those of B stars that exhibit alarge scatter in the LX - LBOL relation, noadditional absorption being found, and the fits indicate a plasma athigher temperatures. Variability (either within one exposure or betweenmultiple exposures) was also investigated whenever possible: short-termvariations are far more rare than long-term ones (the former affects afew percent of the sample, while the latter concerns between one thirdand two thirds of the sources). Conclusions: This paper presentsthe results of the first high-sensitivity investigation of the overallhigh-energy properties of a sizable sample of hot stars.Based on observations collected with XMM-Newton, an ESA Science Missionwith instruments and contributions directly funded by ESA Member Statesand the USA (NASA), and accessed via the 2XMMi and XMM slew surveycatalogs. Tables 1 and 5 are only available in electronic form at theCDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/506/1055 PostdoctoralResearcher FNRS. Visiting astronomer, UNAM-Morelos (Mexico).
| On the presence and absence of disks around O-type stars
As the favoured progenitors of long-duration gamma-ray bursts, massivestars may represent our best signposts of individual objects in theearly Universe, but special conditions seem required to make thesebursters. These are thought to originate from the progenitor's rapidrotation and associated asymmetry.
| Spectral classification of the mass donors in the high-mass X-ray binaries EXO 1722-363 and OAO 1657-415
Aims: We report near-infrared (NIR) observations of the mass donors ofthe eclipsing high-mass X-ray binary (HMXB) systems EXO1722-363 and OAO 1657-415 in order toderive their accurate spectral classifications. Methods: ESO/VLTobservations of the targets with the NIR spectrometer ISAAC werecompared with several published NIR spectral atlases of O and Bsupergiants, an identification of each object's spectral characteristicswas made, enabling the refinement of spectral classification of the massdonors. Results: We determined that EXO1722-363 was of spectral type B0-B1Ia, positioned at adistance 8.0-2.0+2.5 kpc with a progenitor mass inthe range 30-40 Mȯ. Luminosity calculations imply thatLX ~ 1035-1037 erg s-1 forthis distance range. We conclude that EXO 1722-363shares many of the properties associated with other X-ray binary B-typesupergiant donors. We found that OAO 1657-415correlates closely with the spectra of a class of transitional objects,the Ofpe/WNL stars, an intermediate evolutionary stage between massive Otype stars leaving the main sequence and evolving into Wolf-Rayets. Dueto the wide range in luminosity displayed by Ofpe/WNL stars,(log(L/Lȯ) ~ 5.3-6.2) distance determinations areproblematic. For OAO 1657-415 we report a distance of4.4 ≤ d ≤ 12 kpc, implying an X-ray luminosity of 1.5 ×1036 ≤ LX ≤ 1037 ergs-1. We have used our new classification of OAO1657-415 to explain the physical processes responsible for itsunique position within the Corbet diagram. Ofpe/WNL stars demonstrate ahigh rate of mass-loss through a dense stellar wind combined with a lowterminal velocity. This combination of wind properties leads to a highaccretion rate and transfer of angular momentum to the neutron star inthis system. We believe this in turn leads to a smaller instantaneousequilibrium spin period with respect to normal OB supergiants.Based on observations carried out at the European Southern Observatoryunder programme ID 081.D-0073(A).
| XID II: Statistical Cross-Association of ROSAT Bright Source Catalog X-ray Sources with 2MASS Point Source Catalog Near-Infrared Sources
The 18,806 ROSAT All Sky Survey Bright Source Catalog (RASS/BSC) X-raysources are quantitatively cross-associated with near-infrared (NIR)sources from the Two Micron All Sky Survey Point Source Catalog(2MASS/PSC). An association catalog is presented, listing the mostlikely counterpart for each RASS/BSC source, the probability Pid that the NIR source and X-ray source are uniquelyassociated, and the probability P no-id that none of the2MASS/PSC sources are associated with the X-ray source. The catalogincludes 3853 high quality (P id>0.98) X-ray-NIR matches,2280 medium quality (0.98 >= P id>0.9) matches, and4153 low quality (0.9 >= P id>0.5) matches. Of the highquality matches, 1418 are associations that are not listed in the SIMBADdatabase, and for which no high quality match with a USNO-A2 opticalsource was presented for the RASS/BSC source in previous work. Thepresent work offers a significant number of new associations withRASS/BSC objects that will require optical/NIR spectroscopy forclassification. For example, of the 6133 P id>0.92MASS/PSC counterparts presented in the association catalog, 2411 haveno classification listed in the SIMBAD database. These 2MASS/PSC sourceswill likely include scientifically useful examples of known sourceclasses of X-ray emitters (white dwarfs, coronally active stars, activegalactic nuclei), but may also contain previously unknown sourceclasses. It is determined that all coronally active stars in theRASS/BSC should have a counterpart in the 2MASS/PSC, and that the uniqueassociation of these RASS/BSC sources with their NIR counterparts thusis confusion limited.
|
Submit a new article
Related links
Submit a new link
Member of following groups:
|
Observation and Astrometry data
| Constellation: | Cygne |
| Right ascension: | 20h32m22.42s |
| Declination: | +41°18'18.9" |
| Apparent magnitude: | 9.248 |
| Distance: | 10000000 parsecs |
| Proper motion RA: | -1.1 |
| Proper motion Dec: | -5.7 |
| B-T magnitude: | 11.031 |
| V-T magnitude: | 9.396 |
Catalogs and designations:
|
INHABITED SKY
