C-rich Circumstellar Envelopes Research Articles

The detection of heavy metals in the circumstellar envelopes of post-AGB stars

A new type of peculiarity -- a splitting or asymmetry of strong absorption lines, is found in the optical spectra of selected post-AGB stars with C-rich circumstellar envelopes. The effect is maximal in BaII lines whose profile is split into two-three components. The particular components of the split absorption lines are shown to be formed in a structured circumstellar envelope, suggesting an efficient dredge-up of the heavy metals produced during the preceding evolution of this star into the envelope. We suspect that the splitting (or asymmetry) of the profiles of strongest absorptions with low excitation potential of the low level can be associated with the kinematic and chemical properties of the circumstellar environment and with type of its morphology.

SiO in C-rich circumstellar envelopes of AGB stars: effects of non-LTE chemistry and grain adsorption

Aims.New SiO multi-transition millimetre line observations of a sample of carbon stars, including J = observations with the APEX telescope, are used to probe the role of non-equilibrium chemistry and the influence of grains in circumstellar envelopes of carbon stars.

Oxygen Chemistry in the Circumstellar Envelope of the Carbon‐Rich Star IRC +10216

In this paper we study the oxygen chemistry in the C-rich circumstellar shells of IRC +10216. The recent discoveries of oxygen-bearing species (water, hydroxyl radical, and formaldehyde) toward this source challenge our current understanding of the chemistry in C-rich circumstellar envelopes. The presence of icy comets surrounding the star or catalysis on iron grain surfaces have been invoked to explain the presence of such unexpected species. This detailed study aims at evaluating the chances of producing O-bearing species in the C-rich circumstellar envelope only by gas-phase chemical reactions. For the hot inner envelope we show that although most of the oxygen is locked in CO near the photosphere (as expected for a C/O ratio greater than 1), for radial distances larger than ~15 stellar radii, species such as H2O and CO2 have a large abundance under the assumption of thermochemical equilibrium. It is also shown how non-LTE chemistry makes the CO → H2O, CO2 transformation predicted in LTE very difficult. Concerning the chemistry in the colder, outer envelope, we show that formaldehyde can be formed through gas-phase reactions. However, in order to form water vapor, it is necessary to include a radiative association between atomic oxygen and molecular hydrogen with quite a high rate constant. The chemical models explain the presence of HCO+ and predict the existence of SO and H2CS (which has been detected in a 3 mm line survey to be published). We have modeled the line profiles of H2CO, H2O, HCO+, SO, and H2CS using a nonlocal radiative transfer model and the abundance profiles predicted by our chemical model. The results have been compared to the observations and discussed.

Identification of IRAS Sources in the Outer Disk of the Galaxy

view Abstract Citations (21) References (40) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Identification of IRAS Sources in the Outer Disk of the Galaxy Jiang, B. W. ; Deguchi, S. ; Hu, J. Y. ; Yamashita, T. ; Nishihara, E. ; Matsumoto, S. ; Nakada, Y. Abstract Near infrared imaging and photometric observations at J, H and K (or K') bands were performed toward 95 IRAS sources in the outer disk of the Galaxy most of which had been searched for SiO maser emission and are candidates for variable late-type stars with cold circumstellar envelopes. Low resolution optical spectroscopic observations were made to 19 stars of them for classifying the spectral types. Thirty-eight of them, for which counterparts were not found at I band brighter than 19mag, were taken near infrared images for identification. All are identified in the near-infrared images brighter than 16 mag at K' band except two that could be in the faint phase of variation when observed. So most of the IRAS late-type stars perhaps have a near-infrared counterpart although many have no optical counterpart. In the near-infrared color-color diagram, the stars with O-rich and C-rich circumstellar envelopes(CSEs) are separated from each other. The stars with C-rich CSE are redder than those with O-rich CSE in general but there are three very red stars in this sample being SiO maser sources, i.e. with O-rich CSE. By combining the near-infrared colors and IRAS color, the two types of circumstellar envelopes are discriminated. 48 percent are stars with C-rich CSE in the entire sample and the number ratios of C-rich to O-rich stars are 14:43 and 32:6 for the optically identified and unidentified groups respectively. This indicates an increasing proportion of C-rich stars with galacto-centric distance and stellar evolutionary stage. The result is applied to explain the low detection rate of SiO maser emission in a similar and bigger sample in the outer disk of the Galaxy. Publication: The Astronomical Journal Pub Date: April 1997 DOI: 10.1086/118345 Bibcode: 1997AJ....113.1315J full text sources ADS | data products SIMBAD (95)

SiO Maser Sources in the Outer Disk of the Galaxy

view Abstract Citations (42) References (59) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS SiO Maser Sources in the Outer Disk of the Galaxy Jiang, B. W. ; Deguchi, S. ; Yamamura, I. ; Nakada, Y. ; Cho, S. H. ; Yamagata, T. Abstract The observations in the 28SiO j = 1-0 V = 1, J = 1-0 V = 2 lines and the 29SiO j = 1-0, V = 0 lines were simultaneously made toward 181 color-selected IRAS sources in the outer disk of the Galaxy (at longitudes between 90° and 250°). The 28SiO lines were detected in 63 sources (56 new) and the 29SiO line in 11 sources (9 new). Most of the detections are optically variable stars, mainly Mira-type variables and a few semiregular variables. The detection rate (35%) is much lower than that of the bulge survey performed by the same observational system the 45 m telescope at Nobeyama Radio Observatory. The lower detection rate of SiO maser emission is partly attributed to the increase of contaminations by stars with C-rich circumstellar envelopes and young stellar objects in the outer disk sample. The kinematical implications of the observational results are also discussed. The luminosity distances to the SiO maser sources are estimated from their IRAS fluxes at 12 μm and colors. By combining the SiO radial velocity and the luminosity distance, the rotation curve of the outer Galactic disk is found to be slightly falling within the Galactocentric distance between 8.5 and 12 kpc. The sources located within 20° from the Galactic anticenter direction are used to check for a peculiar motion of the local standard of rest. Publication: The Astrophysical Journal Supplement Series Pub Date: October 1996 DOI: 10.1086/192345 Bibcode: 1996ApJS..106..463J Keywords: GALAXY: KINEMATICS AND DYNAMICS; ISM: MOLECULES; MASERS; STARS: VARIABLES: OTHER LONG-PERIOD VARIABLES full text sources ADS | data products SIMBAD (182) CDS (2) Related Materials (1) Catalog: 1997yCat..21060463J

Al-26 and circumstellar envelopes

The effects of the radioactive decay of Al-26 on the circumstellar envelopes of asymptotic giant branch stars are analyzed. The gamma-rays emitted by the product nucleus Mg-26 escape most envelopes, but the beta-decay positrons are stopped and can ionize and heat the gas. The ionization may produce observable effects in C-rich circumstellar envelopes, particularly if the photospheric Al-26 abundance is as large as inferred from measurements of live Al-26 in the primitive solar nebula or the observations of interstellar 1.8 MeV gamma-rays. For the nearby carbon star IRC +10216, the measured abundance of the molecular ion HCO(+) provides an upper limit of about 4 x 10(exp -3) for the photospheric Al-26/Al-27 ratio, consistent with presolar SiC grains with about the same C-12/C-13 ratio.

The discrimination between O- and C-rich circumstellar envelopes from molecular observations

view Abstract Citations (22) References (8) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS The Discrimination between O- and C-rich Circumstellar Envelopes from Molecular Observations Bujarrabal, V. ; Fuente, A. ; Omont, A. Abstract We have observed a wide sample of circumstellar envelopes around evolved stars in 10 radio lines of the molecules HCN, HNC, SiS, HC3N, CS, SiO, SO, and (13)CO. The species form two groups: O-like molecules (SiO and SO, which are relatively stronger in O-rich stars) and C-like molecules (HCN, HNC, SiS, HC3N, and CS, which are stronger in C-rich sources). We find statistically significant criteria that, from the line intensity ratios of O-/C-like molecules, allow us to classify evolved stars as O- or C-rich. Such criteria are also useful to establish a meaningful comparison between peculiar chemistry objects and the main chemical classes. For line pairs including (12)CO or (13)CO, the criteria can also be settled, but the discrimination is less significant. Some observations of S-type stars show that these objects present line intensity properties that are almost exactly equidistant between those of C- and O-rich sources. Publication: The Astrophysical Journal Pub Date: January 1994 DOI: 10.1086/187184 Bibcode: 1994ApJ...421L..47B Keywords: Data Processing; Molecular Spectra; Planetary Nebulae; Radio Spectra; Stellar Envelopes; Brightness; Frequencies; Infrared Astronomy Satellite; Probability Distribution Functions; Radio Telescopes; Astrophysics; RADIO LINES: STARS; STARS: CHEMICALLY PECULIAR; STARS: CIRCUMSTELLAR MATTER; STARS: LATE-TYPE full text sources ADS | data products SIMBAD (47)

HCN, The First Strong Maser in Carbon-Rich Stars

Maser emission has been observed at the frequency of the (0,2,0) J - 1 - 0 line of HCN (89088 MHz, energy 2050 K above the ground state) in 7 C-rich circumstellar envelopes: CIT 6, S Cep, IRC+ 50096, IRC+30374, FX Ser, AFGL 2513 and IRAS 17581-1744. This is the second molecule showing strong maser emission in millimeter lines, and the first strong maser ever observed in a C-rich circumstellar envelope. The emission is particularly strong (70 Jy) in CIT 6 because of its proximity (Guilloteau, Omont, and Lucas 1987). The masers in the stars have luminosities and linewidths (FWHM about 1 km/s) similar to CIT 6; they are also blueshifted by a few km/s with respect to the velocity of the star given by the central velocity of the ground-state HCN emission (Lucas, Guilloteau, and Omont 1987). These properties are somewhat similar to those of SiO masers in O-rich stars, although the intensities of HCN masers are weaker.