The mass loss of C-rich giants

(Shortened version): The mass loss rates, expansion velocities and dust-to-gas density ratios from millimetric observations of 119 carbon-rich giants are compared, as functions of stellar parameters, to the predictions of recent hydrodynamical models. Distances and luminosities previously estimated from HIPPARCOS data, masses from pulsations and C/O abundance ratios from spectroscopy, and effective temperatures from a new homogeneous scale, are used. Predicted and observed mass loss rates agree fairly well, as functions of effective temperature.(shortened). Four stars with detached shells, i.e. episodic strong mass loss, and five cool infrared carbon-rich stars with optically-thick dust shells, have mass loss rates much larger than predicted values.(shortened). Recent drift models can contribute to minimize the discrepancy since they include more dust. Simple approximate formulae are proposed.Comment: 12 pages, 2 figures, electronic table 3 (2 files included in the archive), A&A, accepted 23 July 200

Elemental nitrogen partitioning in dense interstellar clouds

Many chemical models of dense interstellar clouds predict that the majority of gas-phase elemental nitrogen should be present as N2, with an abundance approximately five orders of magnitude less than that of hydrogen. As a homonuclear diatomic molecule, N2 is difficult to detect spectroscopically through infrared or millimetre-wavelength transitions so its abundance is often inferred indirectly through its reaction product N2H+. Two main formation mechanisms each involving two radical-radical reactions are the source of N2 in such environments. Here we report measurements of the low temperature rate constants for one of these processes, the N + CN reaction down to 56 K. The effect of the measured rate constants for this reaction and those recently determined for two other reactions implicated in N2 formation are tested using a gas-grain model employing a critically evaluated chemical network. We show that the amount of interstellar nitrogen present as N2 depends on the competition between its gas-phase formation and the depletion of atomic nitrogen onto grains. As the reactions controlling N2 formation are inefficient, we argue that N2 does not represent the main reservoir species for interstellar nitrogen. Instead, elevated abundances of more labile forms of nitrogen such as NH3 should be present on interstellar ices, promoting the eventual formation of nitrogen-bearing organic molecules.Comment: Accepted for publication in the Proceedings of the National Academy of Sciences of the United States of America - published online since June 11, 201

Structure of the Large Magellanic Cloud from 2MASS

We derive structural parameters and evidence for extended tidal debris from star count and preliminary standard candle analyses of the Large Magellanic Cloud based on Two Micron All Sky Survey (2MASS) data. The full-sky coverage and low extinction in K_s presents an ideal sample for structural analysis of the LMC. The star count surface densities and deprojected inclination for both young and older populations are consistent with previous work. We use the full areal coverage and large LMC diameter to Galactrocentric distance ratio to infer the same value for the disk inclination based on perspective. A standard candle analysis based on a sample of carbon long-period variables (LPV) in a narrow color range, 1.6<J-K_s<1.7 allows us to probe the three-dimensional structure of the LMC along the line of sight. The intrinsic brightness distribution of carbon LPVs in selected fields implies that \sigma_M\simlt 0.2^m for this color cut. The sample provides a {\it direct} determination of the LMC disk inclination: $42.3^\circ\pm 7.2^\circ$. Distinct features in the photometric distribution suggest several distinct populations. We interpret this as the presence of an extended stellar component of the LMC, which may be as thick as 14 kpc, and intervening tidal debris at roughly 15 kpc from the LMC.Comment: 24 pages, 9 figures. Submitted to Ap

Hipparcos period-luminosity relations for Miras and semiregular variables

We present period-luminosity diagrams for nearby Miras and semiregulars, selecting stars with parallaxes better than 20 per cent and well-determined periods. Using K-band magnitudes, we find two well-defined P-L sequences, one corresponding to the standard Mira P-L relation and the second shifted to shorter periods by a factor of about 1.9. The second sequence only contains semiregular variables, while the Mira sequence contains both Miras and semiregulars. Several semiregular stars show double periods in agreement with both relations. The Whitelock evolutionary track is shown to fit the data, indicating that the semiregulars are Mira progenitors. The transition between the two sequences may correspond to a change in pulsation mode or to a change in the stellar structure. Large amplitude pulsations leading to classical Mira classification occur mainly near the tip of the local AGB luminosity function.Comment: 10 pages with figures, accepted by ApJ Letter

Review of important reactions for the nitrogen chemistry in the interstellar medium

Predictions of astrochemical models depend strongly on the reaction rate coefficients used in the simulations. We reviewed a number of key reactions for the chemistry of nitrogen-bearing species in the dense interstellar medium and proposed new reaction rate coefficients for those reactions. The details of the reviews are given in the form of a datasheet associated with each reaction. The new recommended rate coefficients are given with an uncertainty and a temperature range of validity and will be included in KIDA (http://kida.obs.u-bordeaux1.fr).Comment: 39 pages, not published in refereed journal, datasheets are given in KID

Outflow dynamics of dust-driven wind models and implications for cool envelopes of PNe

The density profiles of cool envelopes of young Planetary Nebulae (PNe) are reminiscent of the final AGB outflow history of the central star, so far as these have not yet been transformed by the hot wind and radiation of the central star. Obviously, the evolution of the mass loss rate of that dust-driven, cool wind of the former giant in its final AGB stages must have shaped these envelopes to some extent. Less clear is the impact of changes in the outflow velocity. Certainly, larger and fast changes would lead to significant complications in the reconstruction of the mass-loss history from a cool envelope's density profile. Here, we analyse the outflow velocity v_{\rm exp} in a consistent set of over 50 carbon-rich, dust-driven and well "saturated" wind models, and how it depends on basic stellar parameters. We find a relation of the kind of v_{\rm exp} \propto (L/M)^{0.6}. By contrast to the vast changes of the mass-loss rate in the final outflow phase, this relation suggest only very modest variations in the wind velocity, even during a thermal pulse. Hence, we conclude that the density profiles of cool envelopes around young PNe should indeed compare relatively well with their recent mass-loss history, when diluted plainly by the equation of continuity.Comment: 6 pages, 6 figures, 1 tabl

s-Process Nucleosynthesis in Carbon Stars

We present the first detailed and homogeneous analysis of the s-element content in Galactic carbon stars of N-type. Abundances of Sr,Y, Zr (low-mass s-elements, or ls) and of Ba, La, Nd, Sm and Ce (high-mass s-elements, hs) are derived using the spectral synthesis technique from high-resolution spectra. The N-stars analyzed are of nearly solar metallicity and show moderate s-element enhancements, similar to those found in S stars, but smaller than those found in the only previous similar study (Utsumi 1985), and also smaller than those found in supergiant post-AGB stars. This is in agreement with the present understanding of the envelope s-element enrichment in giant stars, which is increasing along the spectral sequence M-->MS-->S-->SC-->C during the AGB phase. We compare the observational data with recent $s$-process nucleosynthesis models for different metallicities and stellar masses. Good agreement is obtained between low mass AGB star models (M < 3 M_o) and s-elements observations. In low mass AGB stars, the 13C(alpha, n)16O reaction is the main source of neutrons for the s-process; a moderate spread, however, must exist in the abundance of 13C that is burnt in different stars. By combining information deriving from the detection of Tc, the infrared colours and the theoretical relations between stellar mass, metallicity and the final C/O ratio, we conclude that most (or maybe all) of the N-stars studied in this work are intrinsic, thermally-pulsing AGB stars; their abundances are the consequence of the operation of third dredge-up and are not to be ascribed to mass transfer in binary systems.Comment: 31 pages, 10 figures, 6 tables. Accepted in Ap

Hipparcos red stars in the HpV_{T2} and VI_C systems

For Hipparcos M, S, and C spectral type stars, we provide calibrated instantaneous (epoch) Cousins $V-I$ color indices using newly derived $HpV_{T2}$ photometry. Three new sets of ground-based Cousins $VI$ data have been obtained for more than 170 carbon and red M giants. These datasets in combination with the published sources of $VI$ photometry served to obtain the calibration curves linking Hipparcos/Tycho $Hp-V_{T2}$ with the Cousins $V-I$ index. In total, 321 carbon stars and 4464 M- and S-type stars have new $V-I$ indices. The standard error of the mean $V-I$ is about 0.1 mag or better down to $Hp\approx9$ although it deteriorates rapidly at fainter magnitudes. These $V-I$ indices can be used to verify the published Hipparcos $V-I$ color indices. Thus, we have identified a handful of new cases where, instead of the real target, a random field star has been observed. A considerable fraction of the DMSA/C and DMSA/V solutions for red stars appear not to be warranted. Most likely such spurious solutions may originate from usage of a heavily biased color in the astrometric processing.Comment: 10 figures, 1 electronic table, accepted in A&