F. Colomer

28 SiO v =1 and v =2 , J = 1-0 maser variability in evolved stars. Eleven years of short spaced monitoring

This paper presents and discusses the final data set of a long-term and short-spaced monitoring of 21 SiO maser sources, mostly evolved stars, carried out in two SiO maser lines at 43 GHz with the Observatorio Astronomico Nacional 13.7 m telescope at the Centro Astronomico de Yebes (Guadalajara, Spain). In most objects, more than 80 spectra per transition over a period of 11 years have been recorded. The new data presented here, previously unpublished, represent nearly 50% of the total SiO data collected in the project. In addition, the availability of optical light curves from the AAVSO for most of the objects during the whole period of the SiO monitoring, ground-based near-IR data for four sources overlapping with 3 to 5 observed SiO periods, and DIRBE near-IR data covering a significant portion of an SiO period in 10 sources, make this data set a unique reference for comparing optical, NIR and SiO variability in order to elucidate the physical mechanisms that pump SiO masers in evolved stars. The basis for the conclusions obtained in this work comes from a numerical time series analysis of the suitable SiO, optical and NIR light curves in regular variables to obtain precise values of the periods and phase lags between the different curves. This analysis shows evidence that in regular variable evolved stars the three types of emission have the same period and that the SiO maxima happen in phase with NIR maxima and with a phase lag typically between 0.05 and 0.20 with respect to optical maxima. We conclude that in these objects the observational evidence presented in this work favors the radiative pumping of SiO masers against the collisional pumping.

High resolution observations of SiO masers: comparing the spatial distribution at 43 and 86 GHz

We present sub-milliarcsecond observations of SiO masers in the late-type stars IRC + 10011 and cCyg. We have used the NRAO Very Long Baseline Array (VLBA) to map the 43 GHz ( v = 1,2 J = 1-0) and the 86 GHz ( v = 1,2 J = 2-1) SiO masers. All the transitions have been imaged except the v = 2 J = 2-1 in IRC + 10011. We report the first VLBI map of the v = 1 J = 2-1 28SiO maser in IRC + 10011 as well as the first VLBA images of SiO masers in an S-type Mira variable, cCyg. In this paper we have focused on the study of the relative spatial distribution of the different obsd. lines. We have found that in some cases the observational results are not reproduced by the current theor. pumping models, either radiative or collisional. In particular, for IRC + 10011, the v = 1 J = 1-0 and J = 2-1 28SiO lines have different spatial distributions and emitting region sizes, the J = 2-1 emission being located in an outer region of the envelope. For cCyg, the distributions also differ, but the sizes of the masing regions are comparable. We suggest that the line overlaps between ro-vibrational transitions of two abundant mol. species, H2O and 28SiO, is a possible explanation for the discrepancies found between the observations and the theor. predictions. We have introduced this overlapping process in the calcns. of the excitation of the SiO mol. We conclude that the line overlaps can strongly affect the excitation of SiO and may reproduce the unexpected observational results for the two sources studied. [on SciFinder (R)]

Submilliarcsecond-resolution mapping of the 43 GHz SiO maser emission in the bipolar post-AGB nebula OH231.8+4.2

We present ∼0.3 milliarcsec-resolution maps of the SiO (v = 2, J = 1-0) maser emission in the bipolar post-AGB nebula OH 231.8+4.2 obtained with the Very Long Baseline Array. These observations have provided for the firsttime the structure and kinematics of the close stellar environment in a proto-Planetary Nebula. Our observations reveal the SiO maser emission arising in several bright spots of less than ∼10 1 3 cm in size forming a structure elongated in the direction perpendicular to the symmetry axis of the nebula. Such a distribution is consistent with an equatorial torus with a radius of ∼6 AU around the central star. A complex velocity gradient is found along the torus, which suggests rotation and infall of material towards the star. The rotation and infalling velocities deduced are of the same order and range between ∼7 and ∼10 km s - 1 . From our data, we estimate the mass of the SiO torus and the central star, as well as a stringent upper limit to the present stellar mass-loss rate.

Water vapor and silicon monoxide maser observations in the protoplanetary nebula OH 231.8+4.2

Context. OH 231.8+4.2 is a well studied preplanetary nebula (pPN) around a binary stellar system that shows a remarkable bipolar outflow. Aims. To study the structure and kinematics of the inner 10-80AU nebular regions probed by SiO and H 2 O maser emission, where the agents of wind collimation are expected to operate, in order to gain insights into the, yet poorly known, processes responsible for the shaping of bipolar pPNe. Methods. We performed high-resolution observations of the H 2 O 6 1,6 -5 2,3 and 28 SiO v = 2, J = 1-0 maser emissions with the Very Long Baseline Array. The absolute position of both emission distributions were recovered using the phase referencing technique, and accurately registered in HST optical images. Results. Maps of both masers were produced and compared. H 2 O maser clumps are found to be distributed in two areas of 20 mas in size spatially displaced by ∼60 milli-arcs along an axis oriented nearly north-south. SiO masers are tentatively found to be placed between the two H 2 O maser emitting regions, probably indicating the position of the Mira component of the system. Conclusions. The SiO maser emission traces an inner equatorial component with a diameter of 12 AU, probably a disk rotating around the M-type star. Outwards, we detect in the H 2 O data a pair of polar caps, separated by 80 AU. We believe that the inner regions of the nebula probably have been altered by the presence of the companion, leading to an equator-to-pole density contrast that may explain the lack of H 2 O masers and strong SiO maser emission in the denser, equatorial regions.

Probing non-polar interstellar molecules through their protonated form: Detection of protonated cyanogen (NCCNH+)

Cyanogen (NCCN) is the simplest member of the series of dicyanopolyynes. It has been hypothesized that this family of molecules can be important constituents of interstellar and circumstellar media, although the lack of a permanent electric dipole moment prevents its detection through radioastronomical techniques. Here we present the first solid evidence of the presence of cyanogen in interstellar clouds through the detection of its protonated form toward the cold dark clouds TMC-1 and L483. Protonated cyanogen (NCCNH+) has been identified through the J = 5 - 4 and J = 10 - 9 rotational transitions using the 40m radiotelescope of Yebes and the IRAM 30m telescope. We derive beam averaged column densities for NCCNH+ of (8.6 ± 4.4) × 1010 cm-2 in TMC-1 and (3.9 ± 1.8) × 1010 cm-2 in L483, which translate to fairly low fractional abundances relative to H2, in the range (1-10) × 10-12. The chemistry of protonated molecules in dark clouds is discussed, and it is found that, in general terms, the abundance ratio between the protonated and non protonated forms of a molecule increases with increasing proton affinity. Our chemical model predicts an abundance ratio NCCNH+/NCCN of ~ 10-4, which implies that the abundance of cyanogen in dark clouds could be as high as (1-10) × 10-8 relative to H2, i.e., comparable to that of other abundant nitriles such as HCN, HNC, and HC3N.

Relative Astrometry of the J = 1-->0, v = 1 and v = 2 SiO Masers toward R Leonis Minoris Using VERA

Oxygen-rich Asymptotic Giant Branch (AGB) stars are intense emitters of SiO and H2O maser lines at 43 (J = 1!0, v = 1 and 2) and 22 GHz, respectively. VLBI observations of the maser emission provide a unique tool to sample the innermost layers of the circumstellar envelopes in AGB stars. Nevertheless, the difficulties in achieving astrometrically aligned v = 1a ndv = 2 SiO maser maps have traditionally prevented a unique interpretation of the observations in terms of the physical underlying conditions, which depend on the nature of the SiO pumping mechanism. We have carried out observations of the SiO and H2O maser emission towards R LMi, using the astrometric capabilities of VERA. Due to the too weak emission of the reference calibrator, we had to develop a special method to accurately relate the coordinates for both transitions. We present relative astrometrically aligned v = 1 and v = 2 J = 1!0 SiO maser maps, at multiple epochs, and discuss the astrophysical results. The incorporation of astrometric information into the maps of SiO masers challenges the weak points in the current theoretical models, which will need further refinements to address the observation results.

28SiO v = 0 J = 1–0 emission from evolved stars

Observations of 28SiO v=0 J=1-0 line emission (7-mm wavelength) from 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 present systematic observations of 28SiO v=0 J=1-0 emission in 28 evolved stars, performed with the 40~m radio telescope of the IGN in Yebes, Spain. We find that the composite core plus plateau profiles are almost always present in O-rich Miras, OH/IR stars, and red supergiants. They are also found in one S-type Mira (

A study of the close environments of evolved stars from SiO masers

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Circumstellar SiO masers in the long-period variable star TXCam

Cyg), as well as 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 of 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. Alternatively, we speculate that the very similar profiles detected in objects that are axisymmetric may be indicative of the systematic presence of a significant axial symmetry in the very inner circumstellar shells around AGB stars; the presence of such symmetry would be independent of the probable weak maser effects in the central spikes.

Six years of short-spaced monitoring of the v =1 and v =2 , J =1 0 28 SiO maser emission in evolved stars

SiO maser emission allows us to study the innermost circumstellar layers around AGB and post-AGB stars, at a few AU from the stellar photosphere, what is crucial to understand the mass loss process in AGB stars and the jet launching mechanisms in post-AGB stars. Observations of SiO masers are also useful to address the question of the pumping mechanism itself, still under debate. In particular, VLBI observations of such emissions enables the study of the innermost shells with extremely high spatial resolution, equivalent (in nearby stars) to a few 10 12 cm, about one tenth of the stellar radius. In this contribution, recent results on this topic obtained by our group are summarized.