SpitzerLimits on Dust Emission and Optical Gas Absorption Variability around Nearby Stars with Edge‐on Circumstellar Disk Signatures

Abstract

We present Spitzer observations and McDonald Observatory Smith Telescope and Anglo-Australian Telescope high spectral resolution optical observations of 4 nearby stars with variable or anomalous optical absorption, likely caused by circumstellar material. The optical observations of CaII and NaI cover a 2.8 year baseline, and extend the long term monitoring of these systems by previous researchers. In addition, mini-surveys of the local interstellar medium (LISM) around our primary targets provide a reconstruction of the intervening LISM along the line of sight. We confirm that the anomalous absorption detected toward alpha Oph is not due to circumstellar material, but to a small filamentary cloud <14.3 pc from the Sun. The three other primary targets, beta Car, HD85905, and HR10 show both short and long term variability, and little of the observed absorption can be attributed to the LISM along the line of sight. The Spitzer observations did not detect infrared excesses. We are able to place upper limits on any possible fractional infrared luminosity, which range from L_IR/L_star < 2-5 10^-6, for our three disk stars. No stable gas absorption component centered at the radial velocity of the star is detected for any of our targets. Based on simple assumptions of the variable gas absorption component, we estimate limits on the circumstellar gas mass causing the variable absorption, which range from 0.4-20 10^-8 M_Earth. These multiwavelength observations place strong limits on any possible circumstellar dust, while confirming variable circumstellar gas absorption, and therefore are interesting targets to explore the origins and evolution of variable circumstellar gas. (abridged)