P. Battinelli

Hubble Space Telescope Imaging of a Peculiar Stellar Complex in NGC 6946

The stellar populations in a stellar complex in NGC 6946 are analyzed on images taken with the Wide Field Planetary Camera 2 on board the Hubble Space Telescope. The complex is peculiar because of its very high density of stars and clusters and semicircular shape. Its physical dimensions are about the same as for the local Gould Belt, but the stellar density is 1-2 orders of magnitude higher. In addition to an extremely luminous, ~15 Myr old cluster discussed in an earlier paper, accounting for about 17% of the integrated V-band light, we identify 18 stellar clusters within the complex with luminosities similar to the brightest open clusters in the Milky Way. The color-magnitude diagram of individual stars in the complex shows a paucity of red supergiants compared to model predictions in the 10-20 Myr age range for a uniform star formation rate. We thus find tentative evidence for a gap in the dispersed star formation history, with a concentration of star formation into a young globular cluster during this gap. Confirmation of this result must, however, await a better understanding of the late evolution of stars in the corresponding mass range (12 M☉). A reddening map based on individual reddenings for 373 early-type stars is presented, showing significant variations in the absorption across the complex. These may be responsible for some of the arclike structures previously identified on ground-based images. We finally discuss various formation scenarios for the complex and the star clusters within it.

The calibration of the metallicity versus C/M relation

This paper presents a critical reassessment of the C/M ratio in the Local Group galaxies that have been surveyed by us in the last few years. We update distances, reddenings and metallicities with particular care to obtain coherent values for each galaxy. A new equation for the bolometric correction of M stars in terms of their (R − I) colours is obtained from Bessell et al. (1998) data. We present a critical discussion on the colour - spectral type relation for M stars which reveals a dramatic lack of reliable data for late M stars. Finally, we show that - when homogeneous data are used - the log(C/M0+) is well correlated to the (Fe/H) of the parent galaxy. Early objective prism surveys of the Milky Way revealed that C stars and late M stars were not spatially distributed in the same way. Blanco (1965) showed that M stars are found pref- erentially inside the Suns orbit while C stars favor the anti- center region. Westerlund (1965) also noticed that late M stars are much more numerous toward the Galactic center. More than ten years later, when deep late-type star surveys of the Magellanic Clouds were initiated, a pronounced difference be- tween galaxies was discovered. Blanco et al. (1978) were the first to point out the tremendous differences between the ratio of the number of C stars and of late M stars in the Galactic bulge, in the Large Magellanic Cloud (LMC) and in the Small Magellanic Cloud (SMC). They rightly suggested that the ra- tios were anti-correlated with the metal abundance. Iben & Renzini (1983) gave two reasons to explain the trend of the C/M ratio with the abundance of the parent pop- ulation. First, the effect of the carbon dredge-up on the stellar spectrum is more pronounced for stars of low metallicity: it is easier to transform an O-rich star into a C-rich star when there is little oxygen to start with. Second, the giant branch of metal- poor systems is steeper, therefore bluer than the metal-rich one, thus there are few if any M or late M stars in very metal-poor systems.

C stars as kinematic probes of the Milky Way disk from 9 to 15 kpc

Context. The availability, from 2MASS, of a large homogeneous sample of Galactic C stars and the recognition that their absolute magnitude can be accurately determined offer the possibility to use them as kinematical probes to investigate motions in the thin or thick disks. Aims. Determine the radial velocities for 70 C stars, a few degrees from the Galactic plane and distributed in longitudes from 60 ◦ to 220 ◦ . Methods. Spectra, with a resolution of 4300, were obtained with the DAO 1.8 m telescope during 6 beautiful nights in October 2006. Results. The rotation velocities of C stars with 60 ◦ <�< 150 ◦ suggest a flat rotation curve to 15 kpc. A number of stars have velocities that do not fit the thin disk rotation. Some of them, toward � = 200 ◦ are most probably members of the Canis Major overdensity. Conclusions. Effort should be made to extend the rotation curve to more that 20 kpc.

Carbon star survey in the Local Group ? VII. NGC 3109 a galaxy without a stellar halo

We present a CFH12K wide field survey of the carbon star population in and around NGC 3109. Carbon stars, the brightest members of the intermediate-age population, were found nearly exclusively in and near the disk of NGC 3109, ruling out the existence of an extensive intermediate-age halo like the one found in NGC 6822. Over 400 carbon stars identified havehMIi= 4:71, confirming the nearly universality of mean magnitude of C star populations in Local Group galaxies. Star counts over the field reveal that NGC 3109 is a truncated disk shaped galaxy without an extensive stellar halo. The minor axis star counts reach the foreground density between 4 0 and 5 0 , a distance that can be explained by an inclined disk rather than a spheroidal halo. We calculate a global C/M ratio of 1:75 0:20, a value expected for such a metal poor galaxy.

The assessment of the near infrared identification of carbon stars. I. The Local Group galaxies WLM, IC 10 and NGC 6822

Context. The selection of AGB C and M stars from NIR colours has been done in recent years using adjustable criteria that are in needs of standardization if one wants to compare, in a coherent manner, properties of various populations. Aims. We intend to assess the NIR colour technique to identify C and M stars. Methods. We compare the NIR colours of several C stars previously identified from spectroscopy or narrow band techniques in WLM, IC 10 and NGC 6822. Results. We demonstrate that very few M stars have (J − K)0 > 1.4 but a non negligible number of C stars are bluer than this limit. Thus, counts of M and C stars based on such limit do not produce pure samples. Conclusions. C/M ratios determined from NIR colours must be regarded as underestimates mainly because the M numbers include many warm C stars and also K stars if no blue limit is considered.

The standard candle aspect of carbon stars

This paper represents the second part of our effort to summarise various aspects of our homogeneous survey of carbon stars in nearby galaxies. Here we investigate the relationships between the size of the C star populations and the luminosity of the parent galaxies. We also explore

Long period variable survey in NGC 6822

Context. The star-formation history of the intermediate-age population of the Local Group dwarf irregular galaxy NGC 6822 may have been influenced by the often discussed interaction with its HI cloud that might have produced the present polar-ring structure. Aims. We investigate the period distribution of the Mira variables in the central region and the spheroid of NGC 6822. Methods. We use the new wide-field near-infrared imager CPAPIR to acquire Ks images of a 32 � × 32 � area, which includes a good part of the eastern spheroid of NGC 6822.

Integrated parameters of star clusters: a comparison of theory and observations

This paper presents integrated magnitudes and colours for synthetic clusters. The integrated parameters have been obtained for the whole cluster population as well as for the main-sequence (MS) population of star clusters. We have also estimated observed integrated magnitudes and colours of the MS population of galactic open clusters, Large Magellanic Cloud (LMC) and Small Magellanic Cloud (SMC) star clusters. It is found that the colour evolution of the MS population of star clusters is not affected by the stochastic fluctuations, however, these fluctuations significantly affect the colour evolution of the whole cluster population. The fluctuations are maximum in (V−I) colour in the age range 6.7 < log(age) < 7.5. Evolution of integrated colours of the MS population of clusters in the Milky Way, LMC and SMC, obtained in the present study is well explained by the present synthetic cluster model. The observed integrated (B−V) colours of the MS population of LMC star clusters having age ≥500 Myr seem to be distributed around the Z= 0.004 model, whereas (V−I) colours are found to be bluer than those predicted by the Z= 0.004 model. The (V−I) versus (B−V) two-colour diagram for the MS population of the Milky Way star clusters shows a fair agreement between the observations and present model, however, the diagrams for LMC and SMC clusters indicate that observed (V−I) colours are relatively bluer. Possible reasons for this anomaly have been discussed. Comparison of the synthetic (U−B) versus (B−V) relation with the observed integrated parameters of the whole cluster population of the Milky Way, LMC and SMC star clusters indicates that the majority of the bluest clusters [(B−V)0 < 0.0] follow the MS population relation. The colour evolution of young Milky Way, LMC and SMC clusters [6.5 ≤ log(age) ≤8.0] also indicates that a large number of young clusters follow the MS population relation. Therefore, in the absence of a careful modelling of stochastic effects, age determination of young star clusters by comparing their integrated colours with whole cluster synthetic colours may yield erroneous results.

The variability of carbon stars in the Sagittarius dwarf spheroidal galaxy

Context. Stars have been forming in massive dwarf spheroidal galaxies continuously for several Gyr. The detection of AGB carbon stars indicates that an intermediate-age population is present. Sagittarius, the most massive, contains some 100 carbon stars. Most of them are probably variable. Aims. Using photometric monitoring we intend to detect long period variables among Sagittarius carbon stars. We should be able to identify mira variables, semi-regular and irregular variables, and to determine their period and age group. Methods. We have obtained K and J images over four semesters. These images are centered on a sample of 27 photometrically identified C stars to follow their variation and determine their periodicity. Results. We have established the period of 14 program stars and have identified 13 miras among them. From their periods we determined that most of them are in the 3−5 Gyr age group. Conclusions. The known miras, distributed over a wide area within Sagittarius dwarf spheroidal galaxy (Sgr), belong to populations of various ages and, most probably, various metallicities.

Young stars in the periphery of NGC 6822

Taking advantage of the wide (42 0 28 0 ) field of the CFH12K camera, we survey the young star population of NGC 6822 to unprecedented radial distances. We determine the age of the young stars by employing Gallarts et al. (1996b) method based on the presence of stars in dierent boxes in the CMD. We find that star formation took place up to 200 Myr ago in a wide area not always matching the current HI cloud. A50 Myr old association is identified, next to the HI void where no young stars are seen.