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Authors
G. Bono
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We discuss the stellar content of the Galactic Center, and in particular, recent estimates of the star formation rate (SFR). We discuss pros and cons of the different stellar tracers and focus our attention on the SFR based on the three classical Cepheids recently discovered in the Galactic Center. We also discuss stellar populations in field and cluster stars and present some preliminary results based on near-infrared photometry of a field centered on the young massive cluster Arches. We also provide a new estimate of the true distance modulus to the Galactic Center and we found 14.49$pm$0.02(standard)$pm$0.10(systematic) mag (7.91$pm0.08pm0.40$ kpc). Current estimate agrees quite well with similar photometric and kinematic distance determinations available in the literature. We also discuss the metallicity gradient of the thin disk and the sharp change in the slope when moving across the edge of the inner disk, the Galactic Bar and the Galactic Center. The difference becomes even more compelling if we take into account that metal abundances are based on young stellar tracers (classical Cepheids, Red Supergiants, Luminous Blue Variables). Finally, we briefly outline the possible mechanisms that might account for current empirical evidence.
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We present preliminary results of our hst Pa$alpha$ survey of the Galactic Center (gc), which maps the central 0.65$times$0.25 degrees around Sgr A*. This survey provides us with a more complete inventory of massive stars within the gc, compared to previous observations. We find 157 Pa$alpha$ emitting sources, which are evolved massive stars. Half of them are located outside of three young massive star clusters near Sgr A*. The loosely spatial distribution of these field sources suggests that they are within less massive star clusters/groups, compared to the three massive ones. Our Pa$alpha$ mosaic not only resolves previously well-known large-scale filaments into fine structures, but also reveals many new extended objects, such as bow shocks and H II regions. In particular, we find two regions with large-scale Pa$alpha$ diffuse emission and tens of Pa$alpha$ emitting sources in the negative Galactic longitude suggesting recent star formation activities, which were not known previously. Furthermore, in our survey, we detect $sim$0.6 million stars, most of which are red giants or AGB stars. Comparisons of the magnitude distribution in 1.90 $mu$m and those from the stellar evolutionary tracks with different star formation histories suggest an episode of star formation process about 350 Myr ago in the gc .
Until recently our knowledge of the Galactic Bulge stellar populations was based on the study of a few low extinction windows. Large photometric and spectroscopic surveys are now underway to map large areas of the bulge. They probe several complex structures which are still to be fully characterized as well as their links with the inner disc, the thick disc and the inner halo. I will review our current, rapidly increasing, knowledge of the bulge stellar populations and the new insight expected towards the Gaia era to disentangle the formation history of the Galactic inner regions.
Previous studies of the Carina region have revealed its complexity and richness as well as a significant number of early-type stars. In many cases, these studies only concentrated on the central region or were not homogeneous. This latter aspect, in particular, is crucial because very different ages and distances for key clusters have been claimed in recent years. The aim of this work is to study in detail an area of the Galactic plane in Carina. We analyze the properties of different stellar populations and focus on a sample of open clusters and their population of YSOs and highly reddened early stars. We also studied the stellar mass distribution in these clusters and the possible scenario of their formation. Finally, we outline the Galactic spiral structure in this direction. We obtained photometric data for six young open clusters located in Carina at l = 291, and their adjacent stellar fields, which we complemented with spectroscopic observations of a few selected targets. We also culled additional information from the literature. Our results provide more reliable estimates of distances, color excesses, masses, and ages of the stellar populations in this direction. We estimate the basic parameters of the studied clusters and find that they identify two overdensities of young stellar populations. We find evidence of PMS populations inside them, with an apparent coeval stellar formation in the most conspicuous clusters. We also discuss apparent age and distance gradients in the direction NW-SE. We study the mass distributions of several clusters in the region. They consistently show a canonical IMF slope. We discover and characterise an abnormally reddened massive stellar population. Spectroscopic observations of ten stars of this latter population show that all selected targets were massive OB stars. Their location is consistent with the position of the Car-Sag spiral arm.
We have calculated synthetic spectra for typical chemical element mixtures (i.e., a standard alpha-enhanced distribution, and distributions displaying CN and ONa anticorrelations) found in the various subpopulations harboured by Galactic globular clusters. From the spectra we have determined bolometric corrections to the standard Johnson-Cousins and Stroemgren filters, and finally predicted colours. These bolometric corrections and colour-transformations, coupled to our theoretical isochrones with the appropriate chemical composition, provide a complete and self-consistent set of theoretical predictions for the effect of abundance variations on the observed cluster CMD. CNO abundance variations affect mainly wavelengths shorter than 400 nm, due to the arise of molecular absorption bands in cooler atmospheres. As a consequence, colour and magnitude changes are largest in the blue filters, independently of using broad or intermediate bandpasses. Colour-magnitude diagrams involving uvy and UB filters (and their various possible colour combinations) are thus the ones best suited to infer photometrically the presence of multiple stellar generations in individual clusters. They are particularly sensitive to variations in the N abundance, with the largest variations affecting the Red Giant Branch (RGB) and lower Main Sequence (MS). BVI diagrams are expected to display multiple sequences only if the different populations are characterized by variations of the C+N+O sum and helium abundance, that lead to changes in luminosity and effective temperature, but leave the flux distribution above 400 nm practically unaffected. A variation of just the helium abundance, up to the level we investigate here, affects exclusively the interior structure of stars, and is largely irrelevant for the atmospheric structure and the resulting flux distribution in the whole wavelength range spanned by our analysis.
We present a $approx 11.5$ year adaptive optics (AO) study of stellar variability and search for eclipsing binaries in the central $sim 0.4$ pc ($sim 10$) of the Milky Way nuclear star cluster. We measure the photometry of 563 stars using the Keck II NIRC2 imager ($K$-band, $lambda_0 = 2.124 text{ } mu text{m}$). We achieve a photometric uncertainty floor of $Delta m_{K} sim 0.03$ ($approx 3%$), comparable to the highest precision achieved in other AO studies. Approximately half of our sample ($50 pm 2 %$) shows variability. $52 pm 5%$ of known early-type young stars and $43 pm 4 %$ of known late-type giants are variable. These variability fractions are higher than those of other young, massive star populations or late-type giants in globular clusters, and can be largely explained by two factors. First, our experiment time baseline is sensitive to long-term intrinsic stellar variability. Second, the proper motion of stars behind spatial inhomogeneities in the foreground extinction screen can lead to variability. We recover the two known Galactic center eclipsing binary systems: IRS 16SW and S4-258 (E60). We constrain the Galactic center eclipsing binary fraction of known early-type stars to be at least $2.4 pm 1.7%$. We find no evidence of an eclipsing binary among the young S-stars nor among the young stellar disk members. These results are consistent with the local OB eclipsing binary fraction. We identify a new periodic variable, S2-36, with a 39.43 day period. Further observations are necessary to determine the nature of this source.
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