28Sio V = 0 J = 1–0 Emission from Evolved Stars
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A&A 589, A74 (2016) Astronomy DOI: 10.1051/0004-6361/201527174 & c ESO 2016 Astrophysics 28SiO v =0J = 1–0 emission from evolved stars P. de Vicente1, V. Bujarrabal2, A. Díaz-Pulido1,C.Albo1,J.Alcolea3,A.Barcia1,L.Barbas1,R.Bolaño1, F. Colomer4, M. C. Diez1,J.D.Gallego1, J. Gómez-González4, I. López-Fernández1 , J. A. López-Fernández1 ,J.A.López-Pérez1, I. Malo1,A.Moreno1, M. Patino1,J.M.Serna1, F. Tercero1, and B. Vaquero1 1 Observatorio de Yebes (IGN), Apartado 148, 19180 Yebes, Spain e-mail: [email protected] 2 Observatorio Astronómico Nacional (OAN-IGN), Apartado 112, 28803 Alcalá de Henares, Spain 3 Observatorio Astronómico Nacional (OAN-IGN), Alfonso XII 3, 28014 Madrid, Spain 4 Instituto Geográfico Nacional, General Ibañez de Ibero 3, 28003 Madrid, Spain Received 11 August 2015 / Accepted 24 February 2016 ABSTRACT Aims. Observations of 28SiO v = 0 J = 1–0 line emission (7-mm wavelength) from asymptotic giant branch (AGB) stars show in some cases peculiar profiles, composed of a central intense component plus a wider plateau. Very similar profiles have been observed in CO lines from some AGB stars and most post-AGB nebulae and, in these cases, they are clearly associated with the presence of conspicuous axial symmetry and bipolar dynamics. We aim to systematically study the profile shape of 28SiO v = 0 J = 1–0 lines in evolved stars and to discuss the origin of the composite profile structure. Methods. We present observations of 28SiO v = 0 J = 1–0 emission in 28 evolved stars, including O-rich, C-rich, and S-type Mira-type variables, OH/IR stars, semiregular long-period variables, red supergiants and one yellow hypergiant. Most objects were observed in several epochs, over a total period of time of one and a half years. The observations were performed with the 40 m radio telescope of the Instituto Geográfico Nacional (IGN) in Yebes, Spain. Results. We find that the composite core plus plateau profiles are systematically present in O-rich Miras, OH/IR stars, and red supergiants. They are also found in one S-type Mira (χ Cyg) and in two semiregular variables (X Her and RS Cnc) that are known to show axial symmetry. In the other objects, the profiles are simpler and similar to those observed in other molecular lines. The composite structure appears in the objects in which SiO emission is thought to come from the very inner circumstellar layers, prior to dust formation. The central spectral feature is found to be systematically composed of a number of narrow spikes, except for X Her and RS Cnc, in which it shows a smooth shape that is very similar to that observed in CO emission. These spikes show a significant (and mostly chaotic) time variation, while in all cases the smooth components remain constant within the uncertainties. The profile shape could come from the superposition of standard wide profiles and a group of weak maser spikes confined to the central spectral regions because of tangential amplification. Alternatively, we speculate that the very similar profiles detected in objects that are known to be conspicuously axisymmetric, such as X Her and RS Cnc, and in O-rich Mira-type stars, such as IK Tau and TX Cam, may be indicative of the systematic presence of a significant axial symmetry in the very inner circumstellar shells around AGB stars; such symmetry would be independent of the presence of weak maser effects in the central spikes. Key words. stars: AGB and post-AGB – circumstellar matter – radio lines: stars 1. Introduction The v = 0 J = 2–1 and higher-J lines have also been ob- served in AGB and post-AGB CSEs. It seems to be a typical ther- SiO rotational line emission is often detected in evolved stars. mal emission, which is much weaker than the v>0 masers and These lines are particularly intense in O-rich asymptotic giant shows wider profiles that are more or less comparable to those branch (AGB) circumstellar envelopes (CSEs), but they are also observed in lines of other standard molecules, such as CO, HCN, detected in C-rich objects and other evolved stars. The low-J vi- etc. (e.g. González Delgado et al. 2003; Bujarrabal et al. 1989; brationally excited (v ≥ 1) lines present intense maser emission Schöier et al. 2011). In O-rich AGB stars, the v = 0 J = 2–1 line in AGB stars, particularly in O-rich Mira-type variables (Buhl probably originates in very inner shells, within about 1015 cm, et al. 1974; Bujarrabal et al. 1987; Alcolea et al. 1990; Jewell as shown by interferometric maps (Lucas et al. 1992), see also et al. 1991; Deguchi et al. 2010; Cho & Kim 2012, etc.). Their González Delgado et al. (2003). In this region, grains are still profiles present some narrow spikes, distributed over a spec- being formed and the velocity gradient is still noticeable, which tral band of a few km s−1. The maser emitting region is very explains the presence of silicon in the gas, not yet fully locked small, consisting of a number of spots in a ring-like structure into grains, as well as the observed triangular line profiles (e.g. with a radius of few stellar radii that are centered on the star Bujarrabal et al. 1989). On the contrary in C-rich AGB stars this (e.g. Diamond et al. 1994; Gonidakis et al. 2013; Desmurs et al. line seems to come from more extended shells and the profiles 2014), this structure is due to the dominant tangential amplifica- are very similar to those of the standard molecules arising from tion. The variability of these masers is very strong and, at least in the bulk of the envelope. regular pulsators, tightly follows the stellar IR variability cycle The v = 0 rotational lines of the rare isotopes, 29SiO (similar to the optical variability cycle, but with a phase shift of and 30SiO, sometimes present spiky profiles that are attributed about 0.1–0.2); see e.g. Pardo et al. (2004). to maser emission. These lines are weaker than 28SiO v>0 Article published by EDP Sciences A74, page 1 of 12 A&A 589, A74 (2016) masers, but are stronger than the 28SiO v = 0 lines (Alcolea & the receiver was upgraded and its location moved to a new po- Bujarrabal 1992). sition in the Nasmyth cabin. Observations from the first epoch, The 28SiO v = 0 J = 1–0 emission from evolved stars, those performed in 2013 and 2014, have a higher receiver noise at 7-mm wavelength, has been scarcely studied so far, since temperature than those in the second epoch. Since the beginning it is relatively weaker than the v>0 masers. In some ob- of 2015 the receiver was improved and the system noise tem- jects, it was found to show very peculiar composite profiles perature dropped approximately by 30 K in both polarization (Jewell et al. 1991) with a wide and smooth plateau and a cen- channels, to about 90 K. tral spiky component that was proposed to be due to weak maser Calibration at 43 GHz is carried out using the chopper wheel emission (Boboltz & Claussen 2004). Remarkably, similar com- method. A hot load at ambient temperature and the sky as cold posite profiles have been observed in standard thermal lines, load were used to calibrate the data. Opacity was estimated the- particularly in CO, coming from a rare class of objects, most oretically using the ATM code (Pardo et al. 2001, 2002)and of which are semiregular variables; see Knapp et al. (1998), weather conditions measured at a metereological station 400 m Libert et al. (2010)andCastro-Carrizo et al. (2010). In con- away from the antenna. The error in the determination of the trast to the case of the majority of AGB shells, in these objects sky opacity was estimated comparing the results from ATM with the whole circumstellar envelope is strongly axisymmetric; it is those obtained from skydip measurements, from which the for- an hourglass-like structure that consists of a slowly expanding ward efficiency of the antenna (90%) was also estimated. In all equatorial disk (at 5kms−1) that is responsible for the central cases the error was always below 10%. We also tested the va- spectral feature, plus a somewhat faster bipolar outflow (expand- lidity of the sky as cold load by comparing the receiver temper- ing at ∼10 km s−1). The axial structure and line profiles found in ature determined using this same chopper wheel method with a these semiregular stars is very similar to what is typical of young hot and cold load method using liquid nitrogen. The difference planetary and protoplanetary nebulae (PNe, PPNe; see CO emis- is also 10% at most. All together, we estimate that the total cali- sion data in e.g. Bujarrabal et al. 2001; Alcolea et al. 2007)al- bration uncertainty of our observations is ±20%. ∗ though in post-AGB nebulae the involved velocities are signif- The conversion between antenna temperature (Ta )andflux icantly higher. The mechanism responsible for the spectacular scales, 0.16 K/Jy, was obtained using observations toward evolution from AGB CSEs to form PPNe and PNe is still un- sources with well-known flux and size, such as the planets der debate. It is thought to involve the effects of a companion on Venus, Mars, Jupiter, and Saturn, which were scheduled in all the inner circumstellar regions (see Blackman & Lucchini 2014, observing runs. The conversion depends mainly on the aper- and references therein). In this scenario, the companion should ture efficiency of the telescope, which we have estimated to also affect (at least) the inner shells in the AGB phase, yielding be 30% ± 10% in an elevation range between 20 and 80 de- some kind of axisymmetric structure.