Kay Justtanont

Probing the mass-loss history of AGB and red supergiant stars from CO rotational line profiles. II. CO line survey of evolved stars: derivation of mass-loss rate formulae

Context. The evolution of intermediate and low-mass stars on the asymptotic giant branch is dominated by their strong dust-driven winds. More massive stars evolve into red supergiants with a similar envelope structure and strong wind. These stellar winds are a prime source for the chemical enrichment of the interstellar medium. Aims: We aim to (1) set up simple and general analytical expressions to estimate mass-loss rates of evolved stars, and (2) from those calculate estimates for the mass-loss rates of the asymptotic giant branch, red supergiant, and yellow hypergiant stars in our galactic sample. Methods: The rotationally excited lines of carbon monoxide (CO) are a classic and very robust diagnostic in the study of circumstellar envelopes. When sampling different layers of the circumstellar envelope, observations of these molecular lines lead to detailed profiles of kinetic temperature, expansion velocity, and density. A state-of-the-art, nonlocal thermal equilibrium, and co-moving frame radiative transfer code that predicts CO line intensities in the circumstellar envelopes of late-type stars is used in deriving relations between stellar and molecular-line parameters, on the one hand, and mass-loss rate, on the other. These expressions are applied to our extensive CO data set to estimate the mass-loss rates of 47 sample stars. Results: We present analytical expressions for estimating the mass-loss rates of evolved stellar objects for 8 rotational transitions of the CO molecule and thencompare our results to those of previous studies. Our expressions account for line saturation and resolving of the envelope, thereby allowing accurate determination of very high mass-loss rates. We argue that, for estimates based on a single rotational line, the CO(2-1) transition provides the most reliable mass-loss rate. The mass-loss rates calculated for the asympotic giant branch stars range from 4 \times 10^-8 M_E¯ yr^-1 up to 8 \times 10^-5 M_E¯ yr^-1. For red supergiants they reach values between 2 \times 10^-7 M_E¯ yr^-1 and 3 \times 10^-4 M_E¯ yr^-1. The estimates for the set of CO transitions allow time variability to be identified in the mass-loss rate. Possible mass-loss-rate variability is traced for 7 of the sample stars. We find a clear relation between the pulsation periods of the asympotic giant branch stars and their derived mass-loss rates, with a levelling off at ~3 \times 10^-5 M_E¯ yr^-1 for periods exceeding 850 days. Conclusions: Appendices are only available in electronic form at http://www.aanda.org

METHANOL ICE IN THE PROTOSTAR GL-2136

We present ground-based spectra in the 10 and 20 mum atmospheric windows of the deeply embedded protostar GL 2136. These reveal narrow absorption features at 9.7 and 8.9 mum, which we ascribe to the CO-stretch and CH3 rock (respectively) of solid methanol in grain mantles. The peak position of the 9.7 mum band implies that methanol is an important ice mantle component (i.e., CH3OH/H2O > 0.5). However, the CH3OH/H2O abundance ratio derived from the observed column densities is only 0.1. This discrepancy suggests that the solid methanol and water ice are located in independent grain components. These independent components may reflect chemical differentiation during grain mantle formation and/or partial outgassing close to the protostar.

Mass loss and rotational CO emission from Asymptotic Giant Branch stars

We present submillimeter observations of rotational transitions of carbon monoxide from J= 2! 1u p to 7! 6 for a sample of Asymptotic Giant Branch stars and red supergiants. It is the first time that the high transitions J = 6 ! 5 and 7! 6 are included in such a study. With line radiative transfer calculations, we aim to determine the mass-loss history of these stars by fitting the CO line intensities. We find that the observed line intensities of the high transitions, including the J= 4! 3 transition, are significantly lower than the predicted values. We conclude that the physical structure of the outflow of Asymptotic Giant Branch stars is more complex than previously thought. In order to understand the observed line intensities and profiles, a physical structure with a variable mass-loss rate and/or a gradient in stochastic gas velocity is required. A case study of the AGB star WX Psc is performed. We find that the CO line strengths may be explained by variations in mass-loss on time scales similar to those observed in the separated arc-like structures observed around post-AGB stars. In addition, a gradient in the stochastic velocity may play a role. Until this has been sorted out fully, any mass loss determinations based upon single CO lines will remain suspect.

Mass loss from OH/IR stars - Models for the infrared emission of circumstellar dust shells

The IR emission of a sample of 24 OH/IR stars is modeled, and the properties of circumstellar dust and mass-loss rate of the central star are derived. It is shown that for some sources the observations of the far-IR emission is well fitted with a lambda exp -1 law, while some have a steeper index of 1.5. For a few sources, the presence of circumstellar ice grains is inferred from detailed studies of the observed 10-micron feature. Dust mass-loss rates are determined from detailed studies for all the stars in this sample. They range from 6.0 x 10 exp -10 solar mass/yr for an optically visible Mira to 2.2 x 10 exp -6 solar mass/yr for a heavily obscured OH/IR star. These dust mass-loss rates are compared to those calculated from IRAS photometry using 12-, 25-, and 60-micron fluxes. The dust mass-loss rates are also compared to gas mass-loss rates determined from OH and CO observations. For stars with tenuous shells, a dust-to-gas ratio of 0.001 is obtained.

Water content and wind acceleration in the envelope around the oxygen-rich AGB star IK Tauri as seen by Herschel/HIFI

During their asymptotic giant branch evolution, low-mass stars lose a significant fraction of their mass through an intense wind, enriching the interstellar medium with products of nucleosynthesis. We observed the nearby oxygen-rich asymptotic giant branch star IK Tau using the high-resolution HIFI spectrometer onboard Herschel. We report on the first detection of (H2O)-O-16 and the rarer isotopologues (H2O)-O-17 and (H2O)-O-18 in both the ortho and para states. We deduce a total water content (relative to molecular hydrogen) of 6.6 x 10(-5), and an ortho-to-para ratio of 3:1. These results are consistent with the formation of H2O in thermodynamical chemical equilibrium at photospheric temperatures, and does not require pulsationally induced non-equilibrium chemistry, vaporization of icy bodies or grain surface reactions. High-excitation lines of (CO)-C-12, (CO)-C-13, (SiO)-Si-28, (SiO)-Si-29, (SiO)-Si-30, HCN, and SO have also been detected. From the observed line widths, the acceleration region in the inner wind zone can be characterized, and we show that the wind acceleration is slower than hitherto anticipated.

Observations of the Circumstellar Water 110→101 and Ammonia 10→00 Lines in IRC +10216 by the Odin Satellite

Submillimeter lines of H2O and NH3 have been detected in the carbon star IRC +10216 (CW Leo) with the Odin submillimeter satellite. The detection of the J = 110 → 101 557 GHz line of ortho-H2O confirms the earlier detection in the same source with SWAS. The detection of the JK = 10 → 00 572 GHz line represents the first observation of the ground-state rotational transition of NH3 in a stellar envelope. By fitting a molecular line transfer model to the observed lines, we derive an ortho-H2O abundance of 2.4 × 10-6, which is consistent with estimates from the SWAS observation. The derived ortho-NH3 abundance of 1 × 10-6 relative to H2 is significantly higher than those derived from 24 GHz inversion transitions and is slightly higher than those from vibrational transitions in the infrared band. The high H2O and NH3 abundances in the carbon-rich star IRC +10216 underscore shortcomings in the conventional gas-phase LTE and non-LTE chemical models.

Outflows from young objects observed with the ISO-LWS - I. Fine structure lines [O I] 63 mu m, [O I] 145 mu m and [C II] 157 mu m

Far infrared fine structure line data from the ISO archive have been extracted for several hundred YSOs and their outflows, including molecular ( CO) outflows, optical jets and Herbig-Haro (HH) objects. Given the importance of these lines to astrophysics, their excitation and transfer ought to be investigated in detail and, at this stage, the reliability of the diagnostic power of the fine structure transitions of OI and CII has been examined. Several issues, such as the extremely small intensity ratios of the oxygen 63 mu m to 145 mu m lines, are still awaiting an explanation. It is demonstrated that, in interstellar cloud conditions, the 145 mu m line is prone to masing, but that this effect is likely an insufficient cause of the line ratio anomaly observed from cold dark clouds. Very optically thick emission could in principle also account for this, but would need similar, prohibitively high column densities and must therefore be abondoned as a viable explanation. One is left with [OI] 63 mu m self absorption by cold and tenuous foreground gas, as has been advocated for distant luminous sources. Recent observations with the submillimeter observatory Odin support this scenario also in the case of nearby dark molecular clouds. On the basis of this large statistical material we are led to conclude that in star forming regions, the [OI] and [CII] lines generally have only limited diagnostic value.

The Mid-Infrared Instrument for the James Webb Space Telescope, I: Introduction

MIRI (the Mid-Infrared Instrument for the James Webb Space Telescope [JWST]) operates from 5 to 28: 5 mu m and combines over this range: (1) unprecedented sensitivity levels; (2) subarcsecond angular resolution; (3) freedom from atmospheric interference; (4) the inherent stability of observing in space; and (5) a suite of versatile capabilities including imaging, low- and medium-resolution spectroscopy (with an integral field unit), and coronagraphy. We illustrate the potential uses of this unique combination of capabilities with various science examples: (1) imaging exoplanets; (2) transit and eclipse spectroscopy of exoplanets; (3) probing the first stages of star and planet formation, including identifying bioactive molecules; (4) determining star formation rates and mass growth as galaxies are assembled; and (5) characterizing the youngest massive galaxies.

W hya through the eye of odin : Satellite observations of circumstellar submillimetre H2O line emission

We present Odin observations of the AGB star W Hya in the ground-state transition of ortho-H2O, 110 − 101 ,a t 557 GHz. The line is clearly of circumstellar origin. Radiative transfer modelling of the water lines observed by Odin and ISO results in a mass-loss rate of (2.5 ± 0.5) × 10 −7 Myr −1 , and a circumstellar H2O abundance of (2.0 ± 1.0) × 10 −3 . The inferred mass-loss rate is consistent with that obtained from modelling the circumstellar CO radio line emission, and also with that obtained from the dust emission modelling combined with a dynamical model for the outflow. The very high water abundance, higher than the cosmic oxygen abundance, can be explained by invoking an injection of excess water from evaporating icy bodies in the system. The required extra mass of water is quite small, on the order of ∼0.1 M⊕.

The widespread occurrence of water vapor in the circumstellar envelopes of carbon-rich asymptotic giant branch stars: first results from a survey with Herschel/HIFI

We report the preliminary results of a survey for water vapor in a sample of eight C stars with large mid-IR continuum fluxes: V384 Per, CIT 6, V Hya, Y CVn, IRAS 15194-5115, V Cyg, S Cep, and IRC+40540. This survey, performed using the HIFI instrument on board the Herschel Space Observatory, entailed observations of the lowest transitions of both ortho-and para-water: the 556.936 GHz 1(10)-1(01) and 1113.343 GHz 1(11)-0(00) transitions, respectively. Water vapor was unequivocally detected in all eight of the target stars. Prior to this survey, IRC+10216 was the only carbon-rich asymptotic giant branch (AGB) star from which thermal water emissions had been discovered, in that case with the use of the Submillimeter Wave Astronomy Satellite (SWAS). Our results indicate that IRC+10216 is not unusual, except insofar as its proximity to Earth leads to a large line flux that was detectable with SWAS. The water spectral line widths are typically similar to those of CO rotational lines, arguing against the vaporization of a Kuiper Belt analog being the general explanation for water vapor in carbon-rich AGB stars. There is no apparent correlation between the ratio of the integrated water line fluxes to the 6.3 mu m continuum flux-a ratio which measures the water outflow rate-and the total mass-loss rate for the stars in our sample.