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Spitzer Limits on Dust Emission and Optical Gas Absorption Variability around Nearby Stars with Edge-on Circumstellar Disk Signatures
We present Spitzer infrared spectroscopic and photometric observationsand McDonald Observatory Smith Telescope and AAT high spectralresolution optical observations of four nearby stars with variable oranomalous optical absorption, likely caused by circumstellar material.The optical observations of Ca II and Na I cover a 2.8 yr baseline andextend the long-term monitoring of these systems by previousresearchers. In addition, minisurveys of the LISM around our primarytargets provide a reconstruction of the intervening LISM along the lineof sight. We confirm that the anomalous absorption detected towardα Oph is not due to circumstellar material, but to a smallfilamentary cloud <14.3 pc from the Sun. The three other primarytargets, β Car, HD 85905, and HR 10, show both short- and long-termvariability, and little of the observed absorption can be attributed tothe LISM along the line of sight. The Spitzer photometry andspectroscopy did not detect infrared excesses. For the three disk stars,we fit the maximum hypothetical infrared excess that is consistent withobserved upper limits. We place upper limits on any possible fractionalinfrared luminosity,LIR/L*<(2-5)×10-6. Thesefractional luminosities are significantly less than that found towardβ Pic but comparable to other debris disks. No stable gasabsorption component centered at the radial velocity of the star isdetected, consistent with no infrared excess detections. Based on simpleassumptions of the variable gas absorption component, we estimate limitson the circumstellar gas mass causing the variable absorption:(0.4-20)×10-8 M⊕. Thesemultiwavelength observations place strong limits on any possiblecircumstellar dust, while confirming variable circumstellar gasabsorption, and therefore are interesting targets to explore the originsand evolution of variable circumstellar gas.
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