Infrared Studies of Epsilon Aurigae in Eclipse

Abstract

We report here on a series of medium resolution spectro-photometric observations of the enigmatic long period eclipsing binary epsilon Aurigae, during its eclipse interval of 2009-2011, using near-infrared spectra obtained with SpeX on the Infrared Telescope Facility (IRTF), mid-infrared spectra obtained with BASS on AOES and IRTF, MIRSI on IRTF, and MIRAC4 on the MMT, along with mid-infrared photometry using MIRSI on IRTF and MIRAC4 on the MMT, plus 1995-2000 timeframe published photometry and data obtained with Denver's TNTCAM2 at WIRO. The goals of these observations included: (1) comparing eclipse depths with prior eclipse data, (2) confirming the reappearance of CO absorption bands at and after mid-eclipse, associated with sublimation in the disk, (3) seeking evidence for any mid-infrared solid state spectral features from particles in the disk, and (4) providing evidence that the externally irradiated disk has azimuthal temperature differences. IR eclipse depths appear similar to those observed during the most recent (1983) eclipse, although evidence for post-mid-eclipse disk temperature increase is present, due to F star heated portions of the disk coming into view. Molecular CO absorption returned 57 days after nominal mid-eclipse, but was not detected at mid-eclipse plus 34 days, narrowing the association with differentially heated sub-regions in the disk. Transient He i 10830A absorption was detected at mid-eclipse, persisting for at least 90 days thereafter, providing a diagnostic for the hot central region. The lack of solid-state features in Spitzer Infrared Spectrograph, BASS, and MIRAC spectra to date suggests the dominance of large particles (micron-sized) in the disk. Based on these observations, mid-infrared studies out of eclipse can directly monitor and map the disk thermal changes, and better constrain disk opacity and thermal conductivity.

INTERFEROMETRY OF ϵ AURIGAE: CHARACTERIZATION OF THE ASYMMETRIC ECLIPSING DISK

“…They discovered that the precision by which the spectroscopic evolution of the eclipse unfolds indicates that the structure of the disk has not appreciably changed on a time scale of at least the last 100 years. Similarly, infrared studies reported by Stencel et al (2011) have confirmed persistent behavior of disk features over at least the last two eclipse cycles.…”

THE INVISIBLE MONSTER HAS TWO FACES: OBSERVATIONS OF ϵ AURIGAE WITH THE HERSCHEL SPACE OBSERVATORY

“…We observed ǫ Aur with the Photodetector Array Camera and Spectrometer (PACS;Poglitsch et al 2010) and Spectral and Photometric Imaging Receiver (SPIRE; Griffin et al 2010) on Herschel (Open Time Cycle-1 program "OT1_hoard_1"). These observations were accomplished on 9-10 Sep 2011 UT (PACS) and 21 Sep 2011 UT (SPIRE), shortly after the end of the optical eclipse in early July 2011 (Stencel et al 2011). The observations consist of 2, 2, and 4 "mini-scan maps" of 10 repetitions in each of the PACS B (B1; 70 µm), G (B2; 110 µm), and R (160 µm) bands, respectively, with each pair at cross-scan angles of 70 and 110 degrees, and 8 "small map" repetitions at 250, 350, and 500 µm for SPIRE.…”

“…Thus, when using a linear orbital ephemeris with zeropoint at mid-eclipse, the disk opposition occurs at φ ≈ 0.6. To determine when the data were obtained relative to the viewing geometry of the binary, we use a linear ephemeris constructed by combining the best orbital period found by Stefanik et al (2010) with the latest eclipse midpoint date found by Stencel et al (2011) (see Table 1). Figure 2 shows an SED of ǫ Aur derived from the one presented in Paper I, but starting in the near-infrared at 1 µm.…”

“…We increased the long wavelength limit compared to Paper I by including both the new Herschel photometry and the 250 GHz radio detection reported by Altenhoff et al (1994). We also differentiated the data listed in Table 1 of Paper I into two groups that were measured at φ ≈ 0.85-0.15 (as in Paper I, the SED excludes data obtained during eclipse, at φ =0.97-0.03; e.g., see Stencel et al 2011) and at φ ≈0.5-0.7. The former data correspond to the cool back side of the disk (facing away from the F star) during pre-and post-eclipse, while the latter were obtained around opposition phase and correspond to the warm front side of the disk (directly facing the F star).…”

“…We established the nature of ǫ Aur as a large (R ≈ 135 R ⊙ ), but low mass (M ≈ 2 M ⊙ ), post-asymptotic giant branch F-type star orbiting a B5 main sequence star surrounded by a large (R ≈ 4 AU), cool (550 K) dust disk. Subsequent to Paper I, a number of authors have further illuminated the ǫ Aur disk (e.g., Stencel et al 2011, Budaj 2011, Chadima et al 2011, Takeuti 2011, highlighting common threads pointing to agreement with the low mass scenario and existence of discrete optically thick and thin disk components. Kloppenborg et al (2010) even obtained near-infrared interferometric images of the disk passing across the face of the F star during eclipse ingress, which have enabled better constraints on the characteristics of the optically thick disk component.…”