Spectroscopic Identification of Faint White Dwarf Candidates in the Praesepe Open Star Cluster

We present spectroscopic observations of the remaining four candidate white dwarfs in Praesepe. All four candidates are quasars with redshifts between 0.8 and 2.8. One quasar, LB 6072, is observed to have a strong metal-line absorption system blueward of the quasar redshift. The lack of additional white dwarfs in Praesepe leaves the total known white-dwarf population of the cluster at five, well below the number expected from commonly-assumed initial mass functions, though several undiscovered cluster WDs may lie in the outer regions of the cluster. All known Praesepe member white dwarfs are concentrated within 0.6 deg of the cluster center, and the radial profile of cluster white dwarfs is quite similar to the profile of massive cluster stars. This profile is mildly inconsistent with that of ~1Mo cluster stars and suggests that the white dwarfs did not receive a velocity kick during the progenitor star's mass loss phases. If complete, the observed Praesepe white dwarf population is consistent with a steeper high-end initial-mass function than commonly assumed, though the calculated slopes are inconsistent with the present-day mass function of Praesepe. Searches for white dwarfs outside the core of Praesepe and further study of the white dwarf populations of additional open clusters is necessary to constrain further the underlying cause of the white dwarf deficit.Comment: 6 pages, 5 figures, uses emulateapj.cls; accepted for publication in the Astronomical Journa

Chemical abundances in seven red giants of NGC 2360 and NGC 2447

Chemical abundances of about fifteen elements from oxygen to europium are measured in seven red giants of the two open clusters NGC 2360 and NGC 2447. The effective temperatures of the giants are determined spectroscopically by taking advantage of their known masses (~2 Mo in NGC 2360 and ~3 Mo in NGC 2447) and bolometric magnitudes. The average iron abundances we obtain for the two clusters are [Fe/H]=0.07 for NGC 2360 and [Fe/H]=0.03 for NGC 2447. Evolutionary stellar model calculations are performed in the mass range 1-4 Mo in order to analyze the surface Na and O abundances predicted after the first dredge-up. The sodium abundance shows a well defined correlation with stellar mass in the 2-3 Mo range. The agreement between our Na abundance determinations in NGC 2360 and our model predictions at 2 Mo is very good. In contrast, the overabundance in one of the three stars in NGC 2447 exceeds that predicted at 3 Mo by ~0.08 dex, which is significant compared to the observational error bars. The effects of core overshooting, convection prescription, metallicity and nuclear reaction rates on the Na surface predictions of our models are investigated. An oxygen deficiency relative to iron by 0.2 dex is measured in our stars, in disagreement with our model predictions. Assuming that the Sun is 0.1-0.3 dex enriched in oxygen relative to neighbor stars could explain the discrepancy.Comment: 11 pages, 8 figures, accepted for publication in Astronomy and Astrophysic

Disentangling the Hercules stream

Using high-resolution spectra of nearby F and G dwarf stars, we have investigated the detailed abundance and age structure of the Hercules stream. We find that the stars in the stream have a wide range of stellar ages, metallicities, and element abundances. By comparing to existing samples of stars in the solar neighbourhood with kinematics typical of the Galactic thin and thick disks we find that the properties of the Hercules stream distinctly separate into the abundance and age trends of the two disks. Hence, we find it unlikely that the Hercules stream is a unique Galactic stellar population, but rather a mixture of thin and thick disk stars. This points toward a dynamical origin for the Hercules stream, probably caused by the Galactic bar.Comment: Accepted for publication in ApJ Letter

The Galactic Kinematics of Mira Variables

The galactic kinematics of Mira variables derived from radial velocities, Hipparcos proper motions and an infrared period-luminosity relation are reviewed. Local Miras in the 145-200day period range show a large asymmetric drift and a high net outward motion in the Galaxy. Interpretations of this phenomenon are considered and (following Feast and Whitelock 2000) it is suggested that they are outlying members of the bulge-bar population and indicate that this bar extends beyond the solar circle.Comment: 7 pages, 2 figure, to be published in Mass-Losing Pulsating Stars and their Circumstellar Matter, Y. Nakada & M. Honma (eds) Kluwer ASSL serie

Population analysis of open clusters: radii and mass segregation

Aims: Based on our well-determined sample of open clusters in the all-sky catalogue ASCC-2.5 we derive new linear sizes of some 600 clusters, and investigate the effect of mass segregation of stars in open clusters. Methods: Using statistical methods, we study the distribution of linear sizes as a function of spatial position and cluster age. We also examine statistically the distribution of stars of different masses within clusters as a function of the cluster age. Results: No significant dependence of the cluster size on location in the Galaxy is detected for younger clusters (< 200 Myr), whereas older clusters inside the solar orbit turned out to be, on average, smaller than outside. Also, small old clusters are preferentially found close to the Galactic plane, whereas larger ones more frequently live farther away from the plane and at larger Galactocentric distances. For clusters with (V - M_V) < 10.5, a clear dependence of the apparent radius on age has been detected: the cluster radii decrease by a factor of about 2 from an age of 10 Myr to an age of 1 Gyr. A detailed analysis shows that this observed effect can be explained by mass segregation and does not necessarily reflect a real decrease of cluster radii. We found evidence for the latter for the majority of clusters older than 30 Myr. Among the youngest clusters (between 5 and 30 Myr), there are some clusters with a significant grade of mass segregation, whereas some others show no segregation at all. At a cluster age between 50 and 100 Myrs, the distribution of stars of different masses becomes more regular over cluster area. In older clusters the evolution of the massive stars is the most prominent effect we observe.Comment: 14 pages, 12 figures, accepted for publication in Astronomy & Astrophysic

Constraining the fundamental parameters of the O-type binary CPD-41degr7733

Using a set of high-resolution spectra, we studied the physical and orbital properties of the O-type binary CPD-41 7733, located in the core of \ngc. We report the unambiguous detection of the secondary spectral signature and we derive the first SB2 orbital solution of the system. The period is 5.6815 +/- 0.0015 d and the orbit has no significant eccentricity. CPD-41 7733 probably consists of stars of spectral types O8.5 and B3. As for other objects in the cluster, we observe discrepant luminosity classifications while using spectroscopic or brightness criteria. Still, the present analysis suggests that both components display physical parameters close to those of typical O8.5 and B3 dwarfs. We also analyze the X-ray light curves and spectra obtained during six 30 ks XMM-Newton pointings spread over the 5.7 d period. We find no significant variability between the different pointings, nor within the individual observations. The CPD-41 7733 X-ray spectrum is well reproduced by a three-temperature thermal mekal model with temperatures of 0.3, 0.8 and 2.4 keV. No X-ray overluminosity, resulting e.g. from a possible wind interaction, is observed. The emission of CPD-41 7733 is thus very representative of typical O-type star X-ray emission.Comment: Accepted by ApJ, 15 pages, 9 figure