Field #3 of the Palomar-Groningen Survey II. Near-infrared photometry of semiregular variables

Near-infrared photometry (JHKL'M) was obtained for 78 semiregular variables (SRVs) in field #3 of the Palomar-Groningen survey (PG3, l=0, b=-10). Together with a sample of Miras in this field a comparison is made with a sample of field SRVs and Miras. The PG3 SRVs form a sequence (period-luminosity & period-colour) with the PG3 Miras, in which the SRVs are the short period extension to the Miras. The field and PG3 Miras follow the same P/(J--K)o relation, while this is not the case for the field and PG3 SRVs. Both the PG3 SRVs and Miras follow the SgrI period-luminosity relation adopted from Glass et al. (1995, MNRAS 273, 383). They are likely pulsating in the fundamental mode and have metallicities spanning the range from intermediate to approximately solar.Comment: 14 pages LaTeX (2 tables, 8 figures), to appear in A&A 338 (1998); minor modifications in tex

The Effective temperature scale of M dwarfs from spectral synthesis

We present a comparison of low-resolution spectra of 60 stars covering the whole M-dwarf sequence. Using the most recent PHOENIX BT-Settl stellar model atmospheres (see paper by F. Allard, in this book) we do a first quantitative compari- son to our observed spectra in the wavelength range 550-950 nm. We perform a first confrontation between models and observations and we assign an effective tempera- tures to the observed M-dwarfs. Teff-spectral type relations are then compared with the published ones. This comparison also aims at improving the models' opacities.Comment: To be published in the on-line version of the Proceedings of Cool Stars 16 (ASP Conference Series) New version with bibliography correcte

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

Accretion by the Galaxy

Cosmology requires at least half of the baryons in the Universe to be in the intergalactic medium, much of which is believed to form hot coronae around galaxies. Star-forming galaxies must be accreting from their coronae. HI observations of external galaxies show that they have HI halos associated with star formation. These halos are naturally modelled as ensembles of clouds driven up by supernova bubbles. These models can fit the data successfully only if clouds exchange mass and momentum with the corona. As a cloud orbits, it is ablated and forms a turbulent wake where cold high-metallicity gas mixes with hot coronal gas causing the prompt cooling of the latter. As a consequence the total mass of HI increases. This model has recently been used to model the Leiden-Argentina-Bonn survey of Galactic HI. The values of the model's parameters that are required to model NGC 891, NGC 2403 and our Galaxy show a remarkable degree of consistency, despite the very different natures of the two external galaxies and the dramatic difference in the nature of the data for our Galaxy and the external galaxies. The parameter values are also consistent with hydrodynamical simulations of the ablation of individual clouds. The model predicts that a galaxy that loses its cool-gas disc for instance through a major merger cannot reform it from its corona; it can return to steady star formation only if it can capture a large body of cool gas, for example by accreting a gas-rich dwarf. Thus the model explains how major mergers can make galaxies "red and dead."Comment: Invited review at "Assembling the Puzzle of the Milky Way", Grand Bornand, April 2011; 6 page

Stellar sources in the ISOGAL intermediate bulge fields

We present a study of ISOGAL sources in the "intermediate" galactic bulge ($|$$l$$|$ $<$ 2$^\circ$, $|$$b$$|$ $\sim$ 1$^\circ$--4$^\circ$), observed by ISOCAM at 7 and 15 $\mu m$. In combination with near-infrared (I, J, K$_{\rm s}$) data of DENIS survey, complemented by 2MASS data, we discuss the nature of the ISOGAL sources, their luminosities, the interstellar extinction and the mass-loss rates. A large fraction of the 1464 detected sources at 15 $\mu m$ are AGB stars above the RGB tip, a number of them show an excess in ([7]-[15])$_{\rm 0}$ and (K$_{\rm s}$-[15])$_{\rm 0}$ colours, characteristic of mass-loss. The latter, especially (K$_{\rm s}$-[15])$_{\rm 0}$, provide estimates of the mass-loss rates and show their distribution in the range 10$^{-8}$ to 10$^{-5}$ M$_{\rm \odot}$/yr.Comment: 16 pages, accepted for publication in Astronomy and Astrophysic

Kinematic groups across the MW disc: insights from models and from the RAVE catalogue

With the advent of the Gaia data, the unprecedented kinematic census of great part of the Milky Way disc will allow us to characterise the local kinematic groups and new groups in different disc neighbourhoods. First, we show here that the models predict a stellar kinematic response to the spiral arms and bar strongly dependent on disc position. For example, we find that the kinematic groups induced by the spiral arm models change significantly if one moves only ~ 0.6 kpc in galactocentric radius, but ~ 2 kpc in azimuth. There are more and stronger groups as one approaches the spiral arms. Depending on the spiral pattern speed, the kinematic imprints are more intense in nearby vicinities or far from the Sun. Secondly, we present a preliminary study of the kinematic groups observed by RAVE. This sample will allow us, for the first time, to study the dependence on Galactic position of the (thin and thick) disc moving groups. In the solar neighbourhood, we find the same kinematics groups as detected in previous surveys, but now with better statistics and over a larger spatial volume around the Sun. This indicates that these structures are indeed large scale kinematic features.Comment: 4 pages, 3 figures, to appear in the proceedings of "Assembling the Puzzle of the Milky Way", Le Grand Bornand (April 17-22, 2011), C. Reyle, A. Robin, M. Schultheis (eds.

Stellar populations in a standard ISOGAL field in the Galactic disk

We aim to identify the stellar populations (mostly red giants and young stars) detected in the ISOGAL survey at 7 and 15micron towards a field (LN45) in the direction l=-45, b=0.0. The sources detected in the survey of the Galactic plane by the Infrared Space Observatory are characterized based on colour-colour and colour-magnitude diagrams. We combine the ISOGAL catalog with the data from surveys such as 2MASS and GLIMPSE. Interstellar extinction and distance are estimated using the red clump stars detected by 2MASS in combination with the isochrones for the AGB/RGB branch. Absolute magnitudes are thus derived and the stellar populations are identified based on their absolute magnitudes and their infrared excess. A standard approach to the analysis of ISOGAL disk observations has been established. We identify several hundred RGB/AGB stars and 22 candidate young stellar objects in the direction of this field in an area of 0.16 deg^2. An over-density of stellar sources is found at distances corresponding to the distance of the Scutum-Crux spiral arm. In addition, we determine mass-loss rates of AGB-stars using dust radiative transfer models from the literature.Comment: 48pages, 38 figures, accepted for publication in A &

Recent star formation in the inner Galactic Bulge seen by ISOGAL. I - Classification of bright mid-IR sources in a test field

Context: The stellar populations in the central region of the Galaxy are poorly known because of the high visual extinction and very great source density in this direction. Aims: To use recent infrared surveys for studying the dusty stellar objects in this region. Methods: We analyse the content of a 20x20 arcmin^2 field centred at (l,b)=(-0.27,-0.06) observed at 7 and 15 microns as part of the ISOGAL survey. These ISO observations are more than an order of magnitude better in sensitivity and spatial resolution than the IRAS observations. The sources are cross-associated with other catalogues to identify various types of objects. We then derive criteria to distinguish young objects from post-main sequence stars. Results: We find that a sample of about 50 young stellar objects and ultra-compact HII regions emerges, out of a population of evolved AGB stars. We demonstrate that the sources colours and spatial extents, as they appear in the ISOGAL catalogue, possibly complemented with MSX photometry at 21 microns, can be used to determine whether the ISOGAL sources brighter than 300 mJy at 15 microns (or [15] < 4.5 mag) are young objects or late-type evolved stars.Comment: 15 pages, 12 figures. Accepted for publication in Astronomy and Astrophysic