Chemical Abundances of M giants in the Galactic Center: a Single Metal-Rich Population with Low [alpha/Fe]

Context. The formation and evolution of the Milky Way bulge is still largely an unanswered question. One of the most essential observables needed in its modelling are the metallicity distribution and the trends of the alpha elements as measured in stars. While Bulge regions beyond R > 50 pc of the centre has been targeted in several surveys, the central part has escaped detailed study due to the extreme extinction and crowding. The abundance gradients from the center are, however, of large diagnostic value. Aims. We aim at investigating the Galactic Centre environment by probing M giants in the field, avoiding supergiants and cluster members. Methods. For 9 field M-giants in the Galactic Centre region, we have obtained high- and low-resolution spectra observed simultaneously with CRIRES and ISAAC on UT1 and UT3 of the VLT. The low-resolution spectra provide a means of determining the effective temperatures, and the high-resolution spectra provide detailed abundances of Fe, Mg, Si, and Ca. Results. We find a metal-rich population at [Fe/H]=+0.11+-0.15 and a lack of the metal-poor population, found further out in the Bulge, corroborating earlier studies. Our [alpha/Fe] element trends, however, show low values, following the outer Bulge trends. A possible exception of the [Ca/Fe] trend is found and needs further investigation. Conclusions. The results of the analysed field M-giants in the Galactic Centre region, excludes a scenario with rapid formation, in which SNIIe played a dominated role in the chemical enrichment of the gas. The metal-rich metallicities together with low alpha-enhancement seems to indicate a bar-like population perhaps related to the nuclear bar.Comment: Accepted for publication in A&

Nonthermal Mg I Emission At 12 Micron From Procyon

We report on stellar Mg i emission at 12 mu m from alpha CMi (Procyon), a star slightly hotter than the Sun. Solar Mg i emission is well known, and its formation was successfully explained in detail by Carlsson et al. Here, for the first time, we successfully model and compare synthetic spectra of the emission lines at 12 mu m with observations of a star other than the Sun. The use of these lines as stellar diagnostics has been anticipated for 10 years or more (see, e.g., Carlsson et al.). We find that the model reproduces the observed emission in Procyon quite well. We expect that high-resolution spectrographs on 8-10 m telescopes will finally be able to exploit these new diagnostics.Swedish Research Councils VRSTINTNSF AST 03-07497Texas Advanced Research ProgramAstronom

Gamma-Ray Burst Spectra and Light Curves as Signatures of a Relativistically Expanding Plasma

Several patterns have been discovered in how some spectral characteristics change during the decaying phase of long (> few s.) GRB pulses. We compare these observed signatures with those expected from a relativistically expanding shell. Within the internal shock model and assuming a short cooling time, we show that the angular dependence in arrival time can explain the general characteristics of long GRB pulses. This includes the pulse shape, with a fast rise and a slower decay, ~ (1+t/tau)^2, and the spectral evolution, which can be described by the hardness-intensity correlation (HIC), with the intensity being proportional to the square of the hardness. A variation of the relevant time scales involved (the angular spreading and the dynamic) can explain the broad, observed dispersion of the HIC index. Reasonable estimates of physical parameters lead to situations where the HIC relation deviates from a pure power law; features that are indeed present in the observations. Depending on the relative values of the rise and decay times of the intrinsic light curve, the spectral/temporal behavior, as seen by an observer, will produce hard-to-soft or tracking pulses. The observed spectrum is a superposition of many intrinsic spectra arriving from different parts of the shell with varying spectral shifts. Therefore, it will be broader than the emitted spectrum and its spectral parameters could have complex relations with the intrinsic ones. Furthermore, we show that the softening of the low-energy power-law index, that has been observed in some pulses, can be explained by geometric effects and does not need to be an intrinsic behavior.Comment: To appear in ApJ, better resolution figures at http://www.astro.su.se/~felix/research.htm

Temperatures and metallicities of M giants in the galactic Bulge from low-resolution K-band spectra

With the existing and upcoming large multi-fibre low-resolution spectrographs, the question arises how precise stellar parameters such as Teff and [Fe/H] can be obtained from low-resolution K-band spectra with respect to traditional photometric temperature measurements. Until now, most of the effective temperatures in galactic Bulge studies come directly from photometric techniques. Uncertainties in interstellar reddening and in the assumed extinction law could lead to large systematic errors. We aim to obtain and calibrate the relation between Teff and the $\rm ^{12}CO$ first overtone bands for M giants in the galactic Bulge covering a wide range in metallicity. We use low-resolution spectra for 20 M giants with well-studied parameters from photometric measurements covering the temperature range 3200 < Teff < 4500 K and a metallicity range from 0.5 dex down to -1.2 dex and study the behaviour of Teff and [Fe/H] on the spectral indices. We find a tight relation between Teff and the $\rm ^{12}CO(2-0)$ band with a dispersion of 95 K as well as between Teff and the $\rm ^{12}CO(3-1)$ with a dispersion of 120 K. We do not find any dependence of these relations on the metallicity of the star, making them relation attractive for galactic Bulge studies. This relation is also not sensitive to the spectral resolution allowing to apply this relation in a more general way. We also found a correlation between the combination of the NaI, CaI and the $\rm ^{12}CO$ band with the metallicity of the star. However this relation is only valid for sub-solar metallicities. We show that low-resolution spectra provide a powerful tool to obtain effective temperatures of M giants. We show that this relation does not depend on the metallicity of the star within the investigated range and is also applicable to different spectral resolution.Comment: 6 pages, accepted for publication in Astronomy&Astrophysic

On the Galactic chemical evolution of sulphur. Sulphur abundances from the [S i] 1082 nm line in giants

Context. The Galactic chemical evolution of sulphur is still under debate. At low metallicities some studies find no correlation between [S/Fe] and [Fe/H], others find [S/Fe] increasing towards lower metallicities, and still others find a combination of the two. Each scenario has different implications for the Galactic chemical evolution of sulphur. Aims. To contribute to the discussion on the Galactic chemical evolution of sulphur by deriving sulphur abundances from non-LTE insensitive spectral diagnostics in Disk and Halo stars with homogeneously determined stellar parameters. Methods. We derive Teff from photometric colours, logg from stellar isochrones and Bayesian estimation, and [Fe/H] and [S/Fe] from spectrum synthesis. We derive [S/Fe] from the [S i] 1082 nm line in 39 mostly cool and metal-poor giants, using 1D LTE MARCS model atmospheres to model our high-resolution NIR spectra obtained with the VLT, NOT and Gemini South telescopes. Results. We derive homogeneous stellar parameters for 29 stars. Our results argue for a chemical evolution of sulphur that is typical for alpha-elements, contrary to some previous studies. Our abundances are systematically higher by about 0.1 dex in comparison to other studies that arrived at similar conclusions using other sulphur diagnostics. Conclusions. We find the [S i] line to be a valuable diagnostic of sulphur abundances in cool giants down to [Fe/H] ~ -2.3. We argue that a homogeneous determination of stellar parameters is necessary, since the derived abundances are sensitive to them. Our results ([S/Fe]) show reasonable agreement with predictions of contemporary models of Galactic chemical evolution. In these models sulphur is predominantly created in massive stars by oxygen burning, and ejected in the ISM during Type II SNe explosions. Systematic differences with previous studies likely fall within modelling uncertainties.Comment: 13 pages, 6 figures. Accepted for publication in A&

Probing the mass loss history of carbon stars using CO line and dust continuum emission

An extensive modelling of CO line emission from the circumstellar envelopes around a number of carbon stars is performed. By combining radio observations and infrared observations obtained by ISO the circumstellar envelope characteristics are probed over a large radial range. In the radiative transfer analysis the observational data are consistently reproduced assuming a spherically symmetric and smooth wind expanding at a constant velocity. The combined data set gives better determined envelope parameters, and puts constraints on the mass loss history of these carbon stars. The importance of dust in the excitation of CO is addressed using a radiative transfer analysis of the observed continuum emission, and it is found to have only minor effects on the derived line intensities. The analysis of the dust emission also puts further constraints on the mass loss rate history. The stars presented here are not likely to have experienced any drastic long-term mass loss rate modulations, at least less than a factor of about 5, over the past thousands of years. Only three, out of nine, carbon stars were observed long enough by ISO to allow a detection of CO far-infrared rotational lines.Comment: 11pages, 7 figures, accepted by A&

Abundances of disk and bulge giants from hi-res optical spectra: II. O, Mg, Ca, and Ti in the bulge sample

Determining elemental abundances of bulge stars can, via chemical evolution modeling, help to understand the formation and evolution of the bulge. Recently there have been claims both for and against the bulge having a different [$\alpha$/Fe] vs. [Fe/H]-trend as compared to the local thick disk possibly meaning a faster, or at least different, formation time scale of the bulge as compared to the local thick disk. We aim to determine the abundances of oxygen, magnesium, calcium, and titanium in a sample of 46 bulge K-giants, 35 of which have been analyzed for oxygen and magnesium in previous works, and compare them to homogeneously determined elemental abundances of a local disk sample of 291 K-giants. We use spectral synthesis to determine both the stellar parameters as well as the elemental abundances of the bulge stars analyzed here. The method is exactly the same as was used for analyzing the comparison sample of 291 local K-giants in Paper I of this series. Compared to the previous analysis of the 35 stars in our sample, we find lower [Mg/Fe] for [Fe/H]>-0.5, and therefore contradict the conclusion about a declining [O/Mg] for increasing [Fe/H]. We instead see a constant [O/Mg] over all the observed [Fe/H] in the bulge. Furthermore, we find no evidence for a different behavior of the alpha-iron trends in the bulge as compared to the local thick disk from our two samples.Comment: Accepted for publication in A&

Modelling CO emission from Mira's wind

We have modelled the circumstellar envelope of {\it o} Ceti (Mira) using new observational constraints. These are obtained from photospheric light scattered in near-IR vibrational-rotational lines of circumstellar CO molecules at 4.6 micron: absolute fluxes, the radial dependence of the scattered intensity, and two line ratios. Further observational constraints are provided by ISO observations of far-IR emission lines from highly excited rotational states of the ground vibrational state of CO, and radio observations of lines from rotational levels of low excitation of CO. A code based on the Monte-Carlo technique is used to model the circumstellar line emission. We find that it is possible to model the radio and ISO fluxes, as well as the highly asymmetric radio-line profiles, reasonably well with a spherically symmetric and smooth stellar wind model. However, it is not possible to reproduce the observed NIR line fluxes consistently with a `standard model' of the stellar wind. This is probably due to incorrectly specified conditions of the inner regions of the wind model, since the stellar flux needs to be larger than what is obtained from the standard model at the point of scattering, i.e., the intermediate regions at approximately 100-400 stellar radii (2"-7") away from the star. Thus, the optical depth in the vibrational-rotational lines from the star to the point of scattering has to be decreased. This can be accomplished in several ways. For instance, the gas close to the star (within approximately 2") could be in such a form that light is able to pass through, either due to the medium being clumpy or by the matter being in radial structures (which, further out, developes into more smooth or shell-like structures).Comment: 18 pages, 3 figures, accepted for publication in Ap