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A Massive Cluster of Red Supergiants at the Base of the Scutum-Crux Arm
We report on the unprecedented red supergiant (RSG) population of amassive young cluster, located at the base of the Scutum-Crux Galacticarm. We identify candidate cluster RSGs based on 2MASS photometry andmedium-resolution spectroscopy. With follow-up high-resolutionspectroscopy, we use CO band-head equivalent width and high-precisionradial velocity measurements to identify a core grouping of 26physically associated RSGs-the largest such cluster known to date. Usingthe stars' velocity dispersion and their inferred luminosities inconjunction with evolutionary models, we argue that the cluster has aninitial mass of ~40,000 Msolar and is therefore among themost massive in the galaxy. Further, the cluster is only a few hundredparsecs away from the cluster of 14 RSGs recently reported by Figer atal.. These two RSG clusters represent 20% of all known RSGs in theGalaxy, and now offer the unique opportunity to study the presupernovaevolution of massive stars, and the blue- to red-supergiant ratio atuniform metallicity. We use GLIMPSE, MIPSGAL, and MAGPIS survey data toidentify several objects in the field of the larger cluster which seemto be indicative of recent regionwide starburst activity at the pointwhere the Scutum-Crux arm intercepts the Galactic bulge. Futureabundance studies of these clusters will therefore permit the study ofthe chemical evolution and metallicity gradient of the Galaxy in theregion where the disk meets the bulge.
| Detection of Water Ice in Edge-on Protoplanetary Disks: HK Tauri B and HV Tauri C
We report the first detection of 3 μm water ice absorption in aprotoplanetary disk. Low-resolution spectroscopy with Subaru IRCS wascarried out for two T Tauri stars with an edge-on disk, HK Tau B and HVTau C. A 3 μm deep water ice absorption (τ>=1) toward bothobjects was detected. Contribution by foreground cloud material to theice absorption should be small, since AV<=3 toward HK TauA and HV Tau A, HV Tau B. Although HV Tau C is reported to have a smallamount of envelope material, its mass is insufficient to produce thelarge optical depth of the detected water ice. In addition, HK Tau Bdoes not have any significant envelope mass. Therefore, the water iceexists inside the disks of these objects. The optical depth profile ofthe water ice absorption in the protoplanetary disks does not show anysignificant difference from those of protostellar sources. The water iceoptical depth for HV Tau C showed a large variation (Δτ=0.59)in two observing epochs separated by 2.32 yr. Assuming the iceabsorption comes from a spherical cloud at 100 AU and Keplerianrotation, it suggests that a Pluto mass gas and dust cloud with a sizeof 1.4 AU may have passed by the line of sight. The lack of variabilityof the continuum level of HV Tau C at the two epochs implies that thedust grains with an icy mantle are segregated from the grains without anicy mantle.
| L' and M' standard stars for the Mauna Kea Observatories Near-Infrared system
We present L' and M' photometry, obtained at the United Kingdom InfraredTelescope (UKIRT) using the Mauna Kea Observatories Near-Infrared(MKO-NIR) filter set, for 46 and 31 standard stars, respectively. The L'standards include 25 from the in-house `UKIRT Bright Standards' withmagnitudes deriving from Elias et al. and observations at the InfraredTelescope Facility in the early 1980s, and 21 fainter stars. The M'magnitudes derive from the results of Sinton and Tittemore. We estimatethe average external error to be 0.015 mag for the bright L' standardsand 0.025 mag for the fainter L' standards, and 0.026 mag for the M'standards. The new results provide a network of homogeneously observedstandards, and establish reference stars for the MKO system, in thesebands. They also extend the available standards to magnitudes whichshould be faint enough to be accessible for observations with moderndetectors on large and very large telescopes.
| JHK Standard Stars on the CIT Photometric System
We present a set of 58 stars with JHK standard values on the CIT systemand with a suitable magnitude range for use with array detectors onsmall- to moderate-size telescopes. Each final value is based on six to47 measures (with a mean of 17) obtained on separate nights with the USNaval Observatory (USNO) NICMOS3 (HgCdTe) camera. The objects include 20primary CIT standards from Elias et al. and 38 secondary sourcesselected from the SAAO and UKIRT standards lists, cover a K-magnituderange between 6.0 and 10.8, and lie north of declination -20°. Thestars were reduced to the CIT system as defined by Elias et al.,producing a USNO system that is identical to the near-infrared CITsystem. This work densifies the original CIT system by nearly a factorof 3 and extends its range by about 3 mag. The SAAO and UKIRT standardsare also compared with the CIT system.
| H II Emission from a Complete Spectroscopic Survey of Be Stars in h and χ Persei
We describe data for emission-line B stars observed in a spectroscopicsurvey of h and χ Persei. The survey is complete to V=12.5 andcovers an area of ~1100 arcmin2 roughly centered on the twoclusters. We detect 32 Be stars; some have low Hα emissionstrength. Seven of these are new identifications; seven others areconfirmations of Be stars previously identified using photometry. Fiveof the observed Be stars show significant Hα profile variationsfrom epoch to epoch. We show that spectral indices yield physicalcharacteristics of the H II emission region. This automatic method isrobust and easily applied to large spectroscopic samples. We inferHα:Hβ flux ratios of 2-5 and observe a linear relationshipbetween Hα emission and J-K color for these stars. We include aHertzsprung-Russell diagram for the B-type stars in the clusters.
| Infrared L-Band Observations of the Trapezium Cluster: A Census of Circumstellar Disks and Candidate Protostars
We report the results of a sensitive near-infrared JHKL imaging surveyof the Trapezium cluster in Orion. We use the JHKL colors to obtain acensus of infrared excess stars in the cluster. Of (391) stars brighterthan 12th magnitude in the K and L bands, 80%+/-7% are found to exhibitdetectable infrared excess on the J-H, K-L color-color diagram.Examination of a subsample of 285 of these stars with published spectraltypes yields a slightly higher infrared excess fraction of 85%. We findthat 97% of the optical proplyds in the cluster exhibit excess in theJHKL color-color diagram indicating that the most likely origin of theobserved infrared excesses is from circumstellar disks. We interpretthese results to indicate that the fraction of stars in the cluster withcircumstellar disks is between 80%-85%, confirming earlier publishedsuggestions of a high disk fraction for this young cluster. Moreover, wefind that the probability of finding an infrared excess around a star isindependent of stellar mass over essentially the entire range of thestellar mass function down to the hydrogen burning limit. Consequently,the vast majority of stars in the Trapezium cluster appear to have beenborn with circumstellar disks and the potential to subsequently formplanetary systems, despite formation within the environment of a richand dense stellar cluster. We identify 78 stars in our samplecharacterized by K-L colors suggestive of deeply embedded objects. Thespatial distribution of these objects differs from that of the rest ofthe cluster members and is similar to that of the dense molecular cloudridge behind the cluster. About half of these objects are detected inthe short wavelength (J and H) bands, and these are found to becharacterized by extreme infrared excess. This suggests that many ofthese sources could be protostellar in nature. If even a modest fraction(i.e., ~50%) of these objects are protostars, then star formation couldbe continuing in the molecular ridge at a rate comparable to that whichproduced the foreground Trapezium cluster.
| The hyperluminous infrared quasar 3C 318 and its implications for interpreting submm detections of high-redshift radio galaxies
We present near-infrared spectroscopy and imaging of the compactsteep-spectrum radio source 3C 318, which shows it to be a quasar atredshift z=1.574 (the z=0.752 value previously reported is incorrect).3C 318 is an IRAS, ISO and SCUBA source so its new redshift makes it themost intrinsically luminous far-infrared (FIR) source in the 3Ccatalogue (there is no evidence of strong gravitational lensingeffects). Its bolometric luminosity greatly exceeds the1013Lsolar level above which an object is said tobe hyperluminous. Its spectral energy distribution (SED) requires thatthe quasar heats the dust responsible for the FIR flux, as is believedto be the case in other hyperluminous galaxies, and contributes (at the>10per cent level) to the heating of the dust responsible for thesubmm emission. We cannot determine whether a starburst makes animportant contribution to the heating of the coolest dust, so evidencefor a high star formation rate is circumstantial, being based on thehigh dust, and hence gas, mass required by its submm detection. We showthat the current submm and FIR data available for the highest-redshiftradio galaxies are consistent with SEDs similar to that of 3C 318. Thisindicates that at least some of this population may be detected in thesubmm because of dust heated by the quasar nucleus, and thatinterpreting submm detection as evidence for very high(>~1000Msolaryr-1) star formation rates may notalways be valid. We show that the 3C 318 quasar is slightly reddened(AV~0.5), the most likely cause of which is SMC-type dust inthe host galaxy. If very distant radio galaxies are reddened in asimilar way then we show that only slightly greater amounts of dustcould obscure the quasars in these sources. We speculate that the lowfraction of quasars amongst the very high redshift (z>~3) objects inlow-frequency radio-selected samples is the result of such obscuration.The highest-z objects might be preferentially obscured because like 3C318 they are inevitably observed very shortly after the jet-triggeringevent, or because their host galaxies are richer in dust and gas atearlier cosmic epochs, or because of some combination of these twoeffects.
| Molecular hydrogen emission in star forming region NGC 7538.
Not Available
| Possible Young Stellar Objects without Detectable CO Emission
Young stellar objects (YSOs) usually appear in molecular clouds asinfrared objects associated with a molecular envelope. Wouterloot andBrand (1989, AAA 50.133.012) searched 1302 IRAS point sources withreliable fluxes at 25, 60, and 100 mu m near to the galactic plane for12CO (J=1-0) emission; 1077 sources were detected. Amongtheir far-infrared sources without detectable CO emission, we found thatat least 18 objects are invisible at optical and near-infraredwavelengths. The infrared spectral indices between 2.2 mu m and 25 mu mcorrespond to those of class I YSOs, and the IRAS colors are similar tothose of the usual YSOs. These peculiar far-infrared objects are highlyconcentrated around the galactic plane and the distances are estimatedto be ~ 1 kpc. Although their distribution is away from molecularclouds, some of them seem to be associated with large dark clouds orweak radio sources. These objects are possible YSOs with low COabundance in the envelopes.
| Design of a polarimeter for the infrared camera OASIS.
A near infrared polarimeter developed for the infrared camera OASIS ofOkayama Observatory, Japan, is introduced. The polarimetric imaging canbe formed in a field of view of φ ≡ 4.1' by attaching thepolarimeter to the OASIS. The polarimeter consists of a rotatableachromatic half-wave plate and a fixed cooled polarizer. Linearpolarization measurements can be carried out via the modulation of thehalf-wave plate. Test observations show that the polarizationefficiencies in the J, H and K wavebands are 86.3%, 97.0% and 97.5%,respectively. Comparisons of OASIS polarimetry with previousobservations are also presented.
| Near infrared polarimetric imaging of NGC 7538.
In the course of a near infrared polarimetric study of star formingregions, the authors have made polarimetric imaging to NGC 7538. Themosaic polarimetric images cover ten infrared sources in the region, andreveal infrared nebulosities and infrared clusters. In this paper, theypresent their observational results in this study, mainly concentratingon the structure of nebulae and their relationship to infrared sources.
| Star-forming regions in blue compact dwarf galaxies
We present near-infrared 2.1-micron continuum and Br gamma and 1-0S(1)line images of three blue compact dwarf galaxies: II Zw 40, NGC 5253,and He 2-10. Comparison of the morphologies of the emission-ine regionsand continuum together with the line ratios shows that in these casesthe starbursts are typified by strongly peaked Br gamma and weak diffuseH2 emission, indicating the presence of one or more compact nuclei, andalso tidal tails. We consider the possible origins of the H2 emissionand conclude that shock excitation in cloud collisions, and hencedynamical processes, such as interactions or mergers, are important. Bycomparing models of the equivalent width of Br gamma to other ageconstraints from the literature, we show that the star formation musthave occurred not only recently but in a short-duration burst. Thecurrent centers of star formation in all three galaxies are of a similarage but, whereas the hotspots in He 2-10 are coeval, we find phasedifferences between those in NGC 5253. We compare the masses and sizesof the star formation sites to those of present-day globular clusters.
| The near-infrared extinction law and limits on the pre-main-sequence population of the Rho Ophiuchi dark cloud
We describe new techniques to measure the NIR extinction law and toplace limits on the premain-sequence stellar population of a dark cloud.We analyze JHK imaging data for the central 1 sq deg of the Rho Ophiuchicloud core and show that nearly all stars projected onto regions of lowCS intensity, ICS 10 K km/s, are background stars. Most sources atlarger CS intensities lie within cloud material. We use the backgroundstars to derive the slope of the NIR extinction law, E(J-H)/E(H-K) =1.57 +/- 0.03. This result is consistent with previous extinction lawsbut has a factor of two to three smaller uncertainty. The new Rho Ophextinction law yields strong constraints on the number of previouslyundiscovered premain-sequence stars in the cloud, 46 +/- 11, and thenumber of previously undiscovered young stars with near-IR excesses, 15+/- 4. Neither limit exceeds the number of known premain-sequence starsin the cloud about 100. Thus, current samples of premain-sequence starsare reasonably complete for K = 14 or less.
| A molecular line and infrared study of NGC 2264 IRS 1.
We present a study of the region around the intermediate-mass youngstellar object NGC 2264 IRS1. This source is embedded in a dense cloudcore. Infrared images in the J, H, and K band show a jet-like structureconnected with IRS1 as well as a second very deeply embedded small starcluster to the southeastern side. IRS1 itself is surrounded by a numberof embedded low-mass stars. We mapped this area in various CStransitions, CO 3->2, some methanol lines and in C^18^O 2->1. Themapping results clearly show a second cloud clump centered at the smallstar cluster. Two molecular outflows were found in the observed region.One flow is oriented along our line of sight and associated with IRS1and the other flow is centered at the small star cluster. Severaladditional spectral line settings were taken at the IRS1 position to getmore accurate constraints on the gas temperature and density. Thesedata, as well as the maps, were analysed with statistical equilibriumexcitation calculations. The best fit results give an uniformtemperature of about 55K in a quite large inner cloud region (1'x1'),with even warmer gas (>70K) present close to IRS1 and the embeddedstar cluster. This warm cloud core is surrounded by extended material ata temperature of 20 to 30K. Radiative transfer models applied fordifferent molecules constrain the mean central density to 2x10^6^cm^-3^.We estimated the beam-averaged total H_2_ column densities at a numberof grid positions around IRS1 from the C^18^O measurements. Using themodeled CS excitation, we can constrain the beam-averaged CS abundancedistribution over the same area. The inferred CS abundance with respectto H_2_ of 1x10^-9^ is nearly constant over the whole region, and thereis no indication for a CS depletion on this scale. Observations of alarge number of other molecules have been obtained as well at the IRS1position. The resulting abundances are compared to those found in otherhigh- and low-mass young stellar objects. The abundances of the organicmolecules H_2_CO and CH_3_OH are somewhat enhanced, whereas those ofsulfur-bearing molecules are much lower. The most strikingcharacteristic of the NGC 2264 chemistry is its nitrogen chemistry,especially the very high abundances of N_2_H^+^ and N_2_. These resultssuggest that NGC 2264 is in a somewhat later evolutionary stage.
| Multiwavelength Observations of Collisional Ring Galaxies.I.Broad-Band Images, Global Properties, and Radial Colors of the Sample Galaxies
This is one of a series of papers discussing the optical, infrared andradio continuum properties of a sample of collisional ring galaxies. Thepresent paper concentrates on the global broad-band optical (B, V and R)and near-IR (J, H and K) images of the galaxies and describe theirglobal properties. An analysis of the colors of the galaxies over avariety of wavelength baselines is described. In the B and V bands, thebluest colors are found in the outer bright ring. The B-V colors of thesample of galaxies are blue, the median value for the sample is B-V =0.60, and V-K= 2.33 mag. The IR morphology of the galaxies is, in mostcases, very similar to that of the B-band data, suggesting that theclumpy appearance of the star formation in the outer rings is real, andnot a result of patchy dust obscuration. Only in one ring (WN1, aSeyfert ring galaxy) was the IR morphology different from the optical,suggesting the presence of significant dust in the disk. In II Hz 4,faint spiral arms are seen within the ring. There is a suggestion thatthe larger rings have redder V-K colors, which may be due to anincreased incidence of nuclear bulges in larger ring galaxies. Radialprofiles of surface brightness and color are presented for fourgalaxies. In all cases, the colors becomes bluer as one proceedsradially outwards, but in two galaxies, the rings redden again outsidethe main ring, suggesting the existence of a red stellar population thatmay have pre-dated the collisions.
| Observations of Paschen alpha in a Complete Sample of Radio Galaxies
We present infrared spectrophotometry of the Paα (n = 4-3)emission line of hydrogen, together with optical spectrophotometry ofHα and Hβ, of a flux-limited sample of 11 radio sources. Thesample consists of all FR II 3CR radio sources with 0.1 <= z <=0.2 and 5^h^ < R.A. < 16^h^ which contains eight narrow-line radiogalaxies (3CR 135, 184.1, 219, 223, 236, 319, 321, and 327), twobroad-line radio galaxies (3CR 234 and 3CR 303), and one quasar (3CR273). The observations were aimed at the detection of obscured broadlines, hidden from our view by dust absorption, as a means of testingtheories of the unification of radio galaxies and quasars. All theobjects except 3CR 236 and 3CR 273 show significant reddening of thenarrow and broad lines, typically of order A_V_ ~ 1.5 for narrow linesand ~3 for broad lines. We detect highly obscured broad-line regions in3CR 184.1, 219, and 223, which appear to be narrow-line objects in theoptical, so these should be reclassified as broad-line radio galaxies.In all cases except 3CR 273 and 3CR 303, the broad lines are reddenedmore than the narrow lines, locating much of the dust responsible forabsorbing the broad-line emission between the broad- and narrow-lineregions. The dereddened line luminosities range up to those oflow-luminosity quasars. The results are broadly consistent with modelswhich seek to unify radio galaxies and quasars through orientation,where an axisymmetric equatorial obscuring region hides the quasarnucleus from view unless the radio axis is pointing close to our line ofsight. These data provide the first opportunity to model thedistribution of broad-line region extinctions in a complete sample,rather than model just the fraction of quasars and radio galaxies. Wedevelop a simple unification model that matches the observeddistribution of extinctions, explains our observations, and makespredictions about the fraction of obscured quasars that will be presentin samples of higher radio luminosity.
| A Butterfly in the Making: Revealing the Near-Infrared Structure of Hubble 12
Abstract image available at:http://adsabs.harvard.edu/abs/1996ApJ...461..288H
| Evidence for an Obscured Quasar in the Giant Radio Galaxy PKS 0634-205
Abstract image available at:http://adsabs.harvard.edu/abs/1995ApJ...454..683S
| Infrared Imaging and Spectroscopy of the Molecular Shock in IC 443
Abstract image available at:http://adsabs.harvard.edu/abs/1995ApJ...454..277R
| The Nuclear Energy Sources Powering Bright Infrared-selected Galaxies
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1995ApJ...447..545A&db_key=AST
| Multiwavelength observations of ring galaxies. 2: Global star formation in ring galaxies
We present optical H-alpha emission-line images and K band near-infraredimages of a sample of collisionally formed ring galaxies. These are usedto determine the distribution of star formation and the distribution ofthe old stellar population, respectively, in the galaxies. Our resultsshow ring galaxies to have similar Hcx luminosities to starburstgalaxies, with star formation being contained almost exclusively in thering. In the larger ring galaxies we observe the ionized hydrogen to lieon the outer edge of the underlying, broader, K band ring. No conclusiveevidence is found in our sample for a true underlying stellar densitywave. Rather, the evidence suggests that either the density waves havepassed into the outer, mainly H I disk of the galaxies or that theprogenitor galaxies were mainly gas-rich low surface brightness objectsand that most of the optical/IR light we now observe has resulted fromrecent star formation in the expanding ring.
| A Near-Infrared Reflection Nebula Associated with NGC 2024 FIR 4
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1995ApJ...440..722M&db_key=AST
| Near Infrared Imaging of Dwarf Ellipticals Irregulars and Blue Compact Galaxies in the Virgo Cluster
New near-IR images are presented for 13 dwarf galaxies in the Virgocluster. Together with previous data these provide a data base of JHKimaging for 26 dwarf ellipticals (dEs), dwarf irregulars (dIs) and bluecompact dwarfs (BCDs). These images show the dIs to be highly asymmetricand unrelaxed, implying that they are dynamically young and unevolved.This is consistent with their blue near-IR and optical-IR colours whichare most easily explained by young stellar populations. The dEs aresymmetrical and apparently relaxed, with very uniform colours indicatingthat they are dominated by old stars. They generally have exponentiallight profiles, but the brighter galaxies tend to exhibit more cuspedlight distributions, similar to the de Vaucouleurs profiles of brightellipticals. The BCDs have moderately asymmetric light profiles, andparadoxically red colours, possibly indicating an intermediate-agestellar population. They are probably dEs which have undergone bursts ofstar formation in the last few X 10^9^ years, whilst the dIs are afundamentally distinct population. Colour gradients are present in manyof the galaxies, invariably in the sense that the nuclei are redder thanthe surrounding galaxy light.
| Sensitive imaging polarimetry of the faint infrared reflection nebula in B5 IRS1
The paper presents sensitive polarimetric images at J, H, and K of thefaint reflection nebulosity in the low-mass young stellar object B5IRS1. Multiepoch photometry, compared with existing data, shows that thenear-IR luminosity of B5 IRS1 has varied significantly between 1983 and1990, declining steadily in near-IR brightness while maintainingvirtually constant near-IR colors. This may suggest an increase ofneutral extinction by grains which are growing in a circumstellar disk,which is speculated to have later accreted into planetesimals. Thenebula is composed of light scattered in a thin, limb-brightened dustshell partly surrounding the blueshifted lobe of the molecular outflowfrom IRS1. An estimate of the density in the scattering dust shellindicates rough pressure equilibrium between the shell and the molecularflow, and a sound-crossing time in the shell close to the dynamical ageof the flow.
| Variation in the near-infrared surface brightness distribution of the bipolar nebula OH 231.8 + 4.2
Near-infrared images of the bipolar nebula OH 231.8 + 4.2 obtained overa 3 yr period suggest that its surface brightness distribution varieswith time. We present a model in which the variations are explained bythe differences in travel times for photons scattered off dust grains atvarious positions along the polar axis of the nebula. This formulationallows us to determine the brightness fluctuations and heliocentricdistance (d) of the embedded source and the inclination of the nebula(i). To explain the observed variations, the central star must possess a2.2-micron amplitude of about 2 mag - larger than determined previouslyfrom aperture photometry and consistent with the classification of thecentral star as a Mira variable, rather than as a supergiant. Weestimated d of about 1300 pc, and i of greater than about 35 deg withthe south lobe further away. Comparison with published 2.2-micronphotometry suggests that the nebula is brighter at maximum and shows alarger variation in integrated surface brightness than measured about 10yr ago. These results demonstrate the potential utility of long-termmonitoring, via near-IR imaging, of reflection nebulae around variablestars.
| The unusual K-band characteristics of the gravitational lens system MG 1131 + 0456
The radio morphology of the gravitational lens system MG 1131 + 0456 isthat of an 'Einstein ring'. Newly obtained K-prime-band images of thesystem reveal the presence of an infrared bright object that has amorphology reminiscent of that seen in the radio. There are two brightobjects embedded in an elliptical halo; these seem to be the infraredcounterparts to the radio morphology. Two nearby objects have no radiocounterparts. All four objects have extremely red optical-infraredcolors: R - K greater than 6. Few extragalactic objects are this red. Aplausible candidate for the source object is an unusually red radiogalaxy.
| A filter for deep near-infrared imaging
The K passband (central wavelength 2.2 microns, FWHM 0.4 micron) is thelongest wavelength standard near-IR passband through which deepground-based imaging is possible. Thermal emission from telescope,instrument, and sky limits the depth to which such imaging can reach byproducing strongly temperature-dependent backgrounds in the range11-13.5 mag/sq arcsec. This paper describes how a passband, which isdenoted as K-prime, located slightly shortward of the standard Kpassband (central wavelength 2.1 microns), yet still within the sameatmospheric window, leads to a significantly lower thermal component ofthe background, reducing the background surface brightness by up to 0.9mag sq arcsec, and thereby allowing deeper imaging to be obtained in thesame integration time. The photometric differences between the K-primefilter and the standard K filter are discussed.
| Color evolution in high-redshift galaxies
The Simultaneous Photometer for Infrared and Visual Light has been usedto observe 40 radio- and 39 optically-selected giant elliptical galaxiesof known redshift in the 0.019-1.6 range. There is no indication in theresults obtained of differences between the colors of radio and nonradiogalaxies, with the exception of H-K in the z=0.2-0.4 range; the H-Kcolor is best fitted by a passively evolving model with little residualstar formation. Some galaxies exhibit strong blueward deviations. Thisbehavior is most easily explained by star formation episodes involvingsmall fractions of the total number of stars.
| Infrared standard stars
The results of an observational program aimed at setting up a network offaint near-infrared standards of sufficient accuracy are reported. Thenetwork covers both northern and southern hemispheres and includesstandards red enough to provide at least a limited check on colortransformations. The standards are set up at J (1.2 micron), H (1.6micron), K (2.2 microns), and L (3.5 microns), and their H2O and COmolecular absorption indices are determined. The problem of colortransformations between observatories is discussed briefly. Allmagnitudes presented are transformed to the natural system defined bythe CIT observations.
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