The Astrosphere of the Asymptotic Giant Branch Star Cit 6

Abstract

We have discovered two extended half-ring structures in a far-ultraviolet image taken with the GALEX satellite of the well-known mass-losing carbon star CIT 6 (RW LMi). The northern (southern) ring is brighter (fainter) with a diameter of ∼ 15 ′ (∼ 18 ′ ). These structures most likely represent the astrosphere resulting from the shock interaction of CIT 6's molecular wind with the Warm Interstellar Medium, as it moves through the latter. These data provide a direct estimate of the size of CIT 6's circumstellar envelope that is a factor ∼20 larger than previous estimates based on CO millimeter-wave line data. We find that CIT 6 has been undergoing heavy mass-loss for at least 93,000 yr and the total envelope mass is 0.29 M ⊙ or larger, assuming a constant mass-loss rate of 3.2 × 10 −6 M ⊙ yr −1 . Assuming that the shock front has reached a steadystate and CIT 6's motion relative to the ISM is in the sky-plane, we measure the termination-shock standoff distance directly from the image and find that CIT 6 is moving at a speed of about 39 (0.17 cm −3 /n ISM ) 1/2 km s −1 through the interstellar medium around it. However, comparisons with published numerical simulations and analytical modelling shows that CIT 6's forward shock (the northern ring) departs from the parabolic shape expected in steady-state. We discuss several possible explanations for this departure.Subject headings: stars: AGB and post-AGB, stars: mass-loss, stars: individual (CIT 6), circumstellar matter, reflection nebulae IntroductionThe carbon-rich AGB star CIT 6 (RW LMi) is probably the most well-studied carbon star after IRC+10216 that is known to be experiencing heavy mass-loss during its Asymptotic Giant Branch (AGB) evolution. Such stars eject large quantities of processed material, enriched with carbon manufactured in their interiors as a result of 3-α nucleosynthesis, into the interstellar medium via extensive dusty molecular winds that operate during the AGB phase. At 400 pc, CIT 6 is somewhat more distant than IRC+10216, and has been extensively observed from radio to optical wavelengths, with a variety of imaging and spectroscopic techniques. The central star is a long-period variable with a period of about 640 days (Alksnis 1995), a bolometric luminosity of about 10 4 L ⊙ , and an average mass-loss rate of 3.2 × 10 −6 M ⊙ yr −1 (Zhang et al. 2009) resulting in a large circumstellar envelope (CSE) expanding at about 18 km s −1 . HST imaging at optical and near-infrared wavelengths reveals the presence of a small, roughly bipolar nebula, suggesting that the object is transitioning into the pre-planetary nebula phase (Schmidt et al. 2002). These authors also found the presence of faint, diffuse arcs 1 ′′ − 4 ′′ from the central star, and suggested that the primary star has a main-sequence companion of spectral type A-F at a separation greater than 40 AU. Recently, Claussen et al. (2011) discovered the presence of multiple, partial circumstellar ring structures in CIT 6 at even larger distances from the center (up to ∼ 8 ′′ ), from their map...

High-Resolution Co Observation of the Carbon Star Cit 6 Revealing the Spiral Structure and a Nascent Bipolar Outflow

“…From the center positions in the four measurements, we calculate the linear proper motion of (−17 ± 4, 16 ± 4) mas yr −1 , which is in accord with those from the direct measures of positional shifts with respect to reference stars in optical and infrared images; (−16 ± 10, 14 ± 10) mas yr −1 (Monnier et al 2000), and (−25 ± 10, 19 ± 10) mas yr −1 (Röser et al 2008). Therefore, CIT 6 is moving to the northwest direction with a projected speed of ∼ 44 km s −1 , which is also consistent with the speed derived from an astrospheric feature at ∼ 15 -18 in far-ultraviolet images (Menten et al 2012;Sahai & Mack-Crane 2014).…”

“…In fact, substantial pPNe may reveal evidence of the shape transition from outer rings and arcs formed in the CSE during the previous AGB phase to the inner region showing bipolar lobes developed by new vigorous ejections (e.g., Egg nebula, Sahai et al 1998). If the ring-like patterns originated from binary orbital motion, the typical dynamical timescales of the observed patterns (100 -1000 years) correspond to the orbital periods of wide binary systems with the companions locating at a few tens to hundred AU.…”

Mass loss of stars on the asymptotic giant branch

“…Our knowledge of the morphology of the dust CSEs has improved significantly due to observations in dust-scattered light (e.g., Mauron et al 2013;Sahai and Mack-Crane 2014), and through observations with the Herschel Space Observatory of thermal dust emission (e.g., Cox et al 2012a). In particular, the latter shows that on the larger scales there are significant departures from sphericity, but these are not necessarily connected to a non-isotropic mass loss.…”

“…3.1.3, but there are also interesting significant deviations from overall spherical symmetry. For instance, there is morphological evidence of the star (and the CSE) moving through the ISM in terms of bow shocks (Cox et al 2012a;Matthews et al 2013;Sahai and Mack-Crane 2014), Fig. 20, left panel.…”

Planetary nebulae with UVIT

“…The corresponding sigma levels are also mentioned. (Sahai & Mack-Crane 2014). Ultraviolet fluorescence spectra of H 2 , as modelled by France et al (2005) with IC 63, show strong emission blueward of the 1608 Å peak, and no emission longward of 1650 Å. Werner and Lyman bands of H 2 extend up to about 900 Å.…”