$\ion{H}{i}$ and CO in the circumstellar environment of the S-type star RS Cancri

Abstract

Context. The history of mass loss during the AGB phase is key to understanding the stellar evolution and the gas and dust replenishment of the interstellar medium. The mass-loss phenomenon presents fluctuations with a wide variety of timescales and spatial scales and requires combining data from multiple tracers. Aims. We study the respective contributions of the central source and of the external medium to the complex geometry of circumstellar ejecta. Methods. This paper presents Plateau de Bure Interferometer and IRAM 30-m telescope CO rotational line observations, along with H i data obtained with the Nançay Radio Telescope for the oxygen-rich semi-regular variable RS Cnc, in order to probe its circumstellar environment on different scales. Results. We detect both the CO(1-0) and the CO(2-1) rotational lines from RS Cnc. The line profiles are composite, comprising two components of half-width ∼2 km s −1 and ∼8 km s −1 , respectively. Whereas the narrow velocity component seems to originate in an equatorial disk in the central part of the CO envelope, the broad component reveals a bipolar structure, with a north-south velocity gradient. In addition, we obtain new H i data on the source and around it in a field of almost 1 square degree. The H i line is centered on v LSR = 7 km s −1 in agreement with CO observations. A new reduction process reveals a complex extended structure in the northwest direction, of estimated size ∼18 , with a position angle (∼310 • ) opposite the direction of the stellar proper motion (∼140 • ). We derive an H i mass of ∼3 × 10 −2 M for this structure. Based on a non spherical simulation, we find that this structure is consistent with arising from the interaction of the star undergoing mass loss at an average rate of ∼10 −7 M yr −1 over ∼2−3 × 10 5 years with the interstellar medium.Conclusions. Using CO and H i lines, we show that the circumstellar environment around RS Cnc includes two related but well separated regions. With CO, we find a bipolar geometry that probably originates from the intrinsic behavior of recent mass-loss processes.With H i, we find a trail of gas, in a direction opposite to the proper motion of RS Cnc lending support to the hypothesis of an interaction with the interstellar medium. This work illustrates the powerful complementarity of CO and H i observations with regard to a more complete description of circumstellar environments around AGB stars.

The multi-scale environment of RS Cancri from CO and H I observations

“…The H i line at 21 cm has proved to be an excellent spatio-kinematic tracer of the external regions of circumstellar shells (e.g., Matthews & Reid 2007). In particular, the H i map of RS Cnc presented by Matthews & Reid shows a 6 -long tail, in a direction opposite to the space motion of the central star, and clearly different from that of the bipolar flow observed in CO at shorter distances (2-10 ) by Libert et al (2010). In such a case the shaping mechanism is thought to be due to the motion of the star relative to the local interstellar medium Matthews et al 2013).…”

“…Summary of previous data RS Cnc was imaged in the CO1−0 and 2−1 lines by Libert et al (2010). An on-the-fly (OTF) map covering a region of 100 × 100 with steps of 4 in RA and 5 in Dec was obtained at the IRAM 30 m telescope.…”

“…Using high spatial resolution data obtained in the CO1−0 and 2−1 lines, Libert et al (2010) have suggested that the composite line-profiles of the semi-regular AGB star RS Cnc probably originate from an axisymmetrical geometry with a slowly expanding equatorial disk and a faster perpendicular bipolar outflow. Other cases of AGB stars with axisymmetrical expanding shells have been identified in a CO mapping survey of AGB stars by CastroCarrizo et al (2010).…”

“…In this paper, we revisit RS Cnc with new high spatial resolution data obtained A&A 565, A54 (2014) Adelman & Dennis (2005); (4) Dumm & Schild (1998); (5) Libert et al (2010); (6) Knapp et al (1998).…”

Detection of Keplerian dynamics in a disk around the post-AGB star AC Herculis

“…Many of them show oblate, even very flat structures, but without any sign of rotation in the observations. This is the case of axisymmetric shells around semiregular variables (e.g., Libert et al 2010;Castro-Carrizo et al 2010) and of flat equatorial structures that are often found in bipolar PPNe (e.g. Alcolea et al 2007;Castro-Carrizo et al 2002, 2012.…”