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&

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

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

Failed Gamma-Ray Bursts: Thermal UV/Soft X-ray Emission Accompanied by Peculiar Afterglows

We show that the photospheres of "failed" Gamma-Ray Bursts (GRBs), whose bulk Lorentz factors are much lower than 100, can be outside of internal shocks. The resulting radiation from the photospheres is thermal and bright in UV/Soft X-ray band. The photospheric emission lasts for about one thousand seconds with luminosity about several times 10^46 erg/s. These events can be observed by current and future satellites. It is also shown that the afterglows of failed GRBs are peculiar at the early stage, which makes it possible to distinguish failed GRBs from ordinary GRBs and beaming-induced orphan afterglows.Comment: 19 pages, 7 figures, accepted for publication in the Astrophysical Journa

Chemical evolution of the Galactic Center

In recent years, the Galactic Center (GC) region (200 pc in radius) has been studied in detail with spectroscopic stellar data as well as an estimate of the ongoing star formation rate. The aims of this paper are to study the chemical evolution of the GC region by means of a detailed chemical evolution model and to compare the results with high resolution spectroscopic data in order to impose constraints on the GC formation history.The chemical evolution model assumes that the GC region formed by fast infall of gas and then follows the evolution of alpha-elements and Fe. We test different initial mass functions (IMFs), efficiencies of star formation and gas infall timescales. To reproduce the currently observed star formation rate, we assume a late episode of star formation triggered by gas infall/accretion. We find that, in order to reproduce the [alpha/Fe] ratios as well as the metallicity distribution function observed in GC stars, the GC region should have experienced a main early strong burst of star formation, with a star formation efficiency as high as 25 Gyr^{-1}, occurring on a timescale in the range 0.1-0.7 Gyr, in agreement with previous models of the entire bulge. Although the small amount of data prevents us from drawing firm conclusions, we suggest that the best IMF should contain more massive stars than expected in the solar vicinity, and the last episode of star formation, which lasted several hundred million years, should have been triggered by a modest episode of gas infall/accretion, with a star formation efficiency similar to that of the previous main star formation episode. This last episode of star formation produces negligible effects on the abundance patterns and can be due to accretion of gas induced by the bar. Our results exclude an important infall event as a trigger for the last starburst.Comment: 10 pages, 8 figures, accepted for publication in MNRA

Fluorine in the solar neighborhood - is it all produced in AGB-stars?

The origin of 'cosmic' fluorine is uncertain, but there are three proposed production sites/mechanisms: AGB stars, $\nu$ nucleosynthesis in Type II supernovae, and/or the winds of Wolf-Rayet stars. The relative importance of these production sites has not been established even for the solar neighborhood, leading to uncertainties in stellar evolution models of these stars as well as uncertainties in the chemical evolution models of stellar populations. We determine the fluorine and oxygen abundances in seven bright, nearby giants with well-determined stellar parameters. We use the 2.3 $\mu$m vibrational-rotational HF line and explore a pure rotational HF line at 12.2 $\mu$m. The latter has never been used before for an abundance analysis. To be able to do this we have calculated a line list for pure rotational HF lines. We find that the abundances derived from the two diagnostics agree. Our derived abundances are well reproduced by chemical evolution models only including fluorine production in AGB-stars and therefore we draw the conclusion that this might be the main production site of fluorine in the solar neighborhood. Furthermore, we highlight the advantages of using the 12 $\mu$m HF lines to determine the possible contribution of the $\nu$-process to the fluorine budget at low metallicities where the difference between models including and excluding this process is dramatic

Mg I emission lines at 12 and 18 micrometer in K giants

The solar Mg I emission lines at 12 micrometer have already been observed and analyzed well. Previous modeling attempts for other stars have, however, been made only for Procyon and two cool evolved stars, with unsatisfactory results for the latter. We present high-resolution observational spectra for the K giants Pollux, Arcturus, and Aldebaran, which show strong Mg I emission lines at 12 micrometer as compared to the Sun. We also present the first observed stellar emission lines from Mg I at 18 micrometer and from Al I, Si I, and presumably Ca I at 12 micrometer. To produce synthetic line spectra, we employ standard non-LTE modeling for trace elements in cool stellar photospheres. We compute model atmospheres with the MARCS code, apply a comprehensive magnesium model atom, and use the radiative transfer code MULTI to solve for the magnesium occupation numbers in statistical equilibrium. We successfully reproduce the observed Mg I emission lines simultaneously in the giants and in the Sun, but show how the computed line profiles depend critically on atomic input data and how the inclusion of energy levels with n > 9 and collisions with neutral hydrogen are necessary to obtain reasonable fits.Comment: 9 pages, 6 figures, accepted for publication in Astronomy & Astrophysic