Studies of multiple stellar systems - III. Modulation of orbital elements in the triple-lined system HD 109648

The triple-lined spectroscopic triple system HD 109648 has one of the shortest periods known for the outer orbit in a late-type triple, 120.5 days, and the ratio between the periods of the outer and inner orbits is small, 22:1. With such extreme values, this system should show orbital element variations over a timescale of about a decade. We have monitored the radial velocities of HD 109648 with the CfA Digital Speedometers for eight years, and have found evidence for modulation of some orbital elements. While we see no definite evidence for modulation of the inner binary eccentricity, we clearly observe variations in the inner and outer longitudes of periastron, as well as in the radial velocity amplitudes of the three components. The observational results, combined with numerical simulations, allow us to put constraints on the orientation of the orbits.Comment: 11 pages, 7 figures, accepted by MNRA

A variable star survey of the open cluster M37

A CCD photometric study of the dense galactic open cluster M37 is presented and discussed. The majority of the analysed data are time-series measurements obtained through an R_C filter. The observations were carried out on seven nights between December 1999 and February 2000, and have led to the discovery of 7 new variable stars in the field. Three of them have been unambiguously identified as W UMa-type eclipsing binaries, while two more are monoperiodic pulsating stars, most probably high-amplitude delta Scuti-type variables. The remaining two stars seem to be long-period eclipsing binaries without firm period determination. Johnson B and V frames have been used to construct a new colour-magnitude (CM) diagram of the cluster, and to find the locations of the new variable stars. The pulsating variables are most likely background objects. The CM diagram is fitted with recent isochrones yielding the main parameters of the cluster.Comment: 7 pages, 6 figures, accepted for publication in A&

Disk or Halo white dwarfs? Kinematic analysis of high proper motion surveys

We present an alternative method for the kinematic analysis of high proper motion surveys and discuss its application to the survey of Oppenheimer et al. (2001) for the selection of reliable halo white dwarfs (WDs). The local WD space density we estimate is (1-2) 10^{-5}MSun/pc^{3}, which is about an order of magnitude smaller than the value derived in Oppenheimer et al. (2001), and is consistent with the values obtained from recent reanalyses of the same data (e.g. Reid et al. 2001, Reyle et al. 2001, Torres et al. 2002, Salim et al. 2004). Our result, which corresponds to a fraction of 0.1%-0.2% of the local dark matter, does not support the scenario suggested by the microlensing experiments that ancient cool WDs could contribute significantly to the dark halo of the Milky Way.Comment: 9 pages, 4 figures, accepted in Astronomy & Astrophysic

Hipparcos red stars in the HpV_{T2} and VI_C systems

For Hipparcos M, S, and C spectral type stars, we provide calibrated instantaneous (epoch) Cousins $V-I$ color indices using newly derived $HpV_{T2}$ photometry. Three new sets of ground-based Cousins $VI$ data have been obtained for more than 170 carbon and red M giants. These datasets in combination with the published sources of $VI$ photometry served to obtain the calibration curves linking Hipparcos/Tycho $Hp-V_{T2}$ with the Cousins $V-I$ index. In total, 321 carbon stars and 4464 M- and S-type stars have new $V-I$ indices. The standard error of the mean $V-I$ is about 0.1 mag or better down to $Hp\approx9$ although it deteriorates rapidly at fainter magnitudes. These $V-I$ indices can be used to verify the published Hipparcos $V-I$ color indices. Thus, we have identified a handful of new cases where, instead of the real target, a random field star has been observed. A considerable fraction of the DMSA/C and DMSA/V solutions for red stars appear not to be warranted. Most likely such spurious solutions may originate from usage of a heavily biased color in the astrometric processing.Comment: 10 figures, 1 electronic table, accepted in A&

Photometry and membership for low mass stars in the young open cluster NGC 2516

We present the results of a 0.86 square degree CCD photometric survey of the open cluster NGC 2516, which has an age of about 150 Myr and may have a much lower metallicity than the similarly-aged Pleiades. We select a preliminary catalogue of 1254 low mass (between 0.2 and 2.0M_{sun}) cluster candidates, of which about 70--80 percent are expected to be genuine. The mass function is metallicity dependent, but consistent with a Salpeter-like law (dN/dlog M ~ M^{-alpha}, alpha=+1.47+/-0.11 or alpha=+1.67+/-0.11 for solar and half-solar metallicities) between 0.7 and 3.0M_{sun}. At lower masses (between 0.3 and 0.7M_{sun}) there is a sharp fall in the mass function, with alpha=-0.75+/-0.20 (solar metallicity) or alpha=-0.49+/-0.13 (half-solar metallicity), which seems inconsistent with the much flatter mass functions seen in the Pleiades and field populations. We explain this by demonstrating that mass segregation has been at work in NGC 2516 -- more than half the cluster low mass stars are expected to lie outside out survey. The mass of NGC 2516 stars with mass greater than 0.3M_{sun} inside our survey is 950-1200M_{sun}, depending on metallicity and what corrections are applied for unresolved binarity. Correcting for mass segregation increases this to ~1240-1560M_{sun}, about twice the total mass of the Pleiades.Comment: 27 pages, accepted for Astronomy & Astrophysic

The old Galactic open clusters FSR1716 and Czernik23

Open clusters older than $\sim4$ Gyr are rare in the Galaxy. Affected by a series of mass-decreasing processes, the stellar content of most open clusters dissolves into the field in a time-scale shorter than $\sim1$ Gyr. In this sense, improving the statistics of old objects may provide constraints for a better understanding of the dynamical dissolution of open clusters. Isochrone fits indicate that FSR 1716 is more probably an old ($\sim7$ Gyr) and absorbed (\aV=6.3\pm0.2) open cluster, located $\approx0.6$ kpc inside the Solar circle in a contaminated central field. However, we cannot rule out the possibility of a low-mass, loose globular cluster. Czernik 23 is shown to be an almost absorption-free open cluster, $\sim5$ Gyr old, located about 2.5 kpc towards the anti-centre. In both cases, Solar and sub-Solar ($[Fe/H]\sim-0.5$) metallicity isochrones represent equally well the stellar sequences. Both star clusters have a low mass content (\la200 \ms) presently stored in stars. Their relatively small core and cluster radii are comparable to those of other open clusters of similar age. These structural parameters are probably consequence of the several Gyrs of mass loss due to stellar evolution, tidal interactions with the disk (and bulge in the case of FSR 1716), and possibly giant molecular clouds. Czernik 23, and especially FSR 1716, are rare examples of extreme dynamical survivors. The identification of both as such represents an increase of $\approx10$ to the known population of open clusters older than $\sim4$ Gyr in the Galaxy.Comment: 15 pages and 13 figures. Accepted by A&

Old open clusters in the inner Galaxy: FSR1744, FSR89 and FSR31

We establish the nature and derive fundamental and structural parameters of the recently catalogued objects FSR1744, FSR89 and FSR31. This work intends to provide clues to constrain the Galactic tidal disruption efficiency, improve statistics of the open cluster parameter space, and better define their age-distribution function inside the Solar circle. Properties of the objects are investigated by means of 2MASS colour-magnitude diagrams and stellar radial density profiles built with field star decontaminated photometry. Diagnostic-diagrams for structural parameters are used to help disentangle dynamical from high-background effects affecting such centrally projected open clusters. FSR1744, FSR89 and FSR31 are Gyr-class OCs located at Galactocentric distances 4.0 - 5.6kpc. Compared to nearby OCs, they have small core and limiting radii. With respect to the small number of OCs observed in the inner Galaxy, the emerging scenario in the near-infrared favours disruption driven by dynamical evolution rather than observational limitations associated with absorption and/or high background levels. Internally, the main processes associated with the dynamical evolution are, e.g. mass loss by stellar evolution, mass segregation and evaporation. Externally they are, e.g. tidal stress from the disk and bulge, and interactions with giant molecular clouds. FSR1744, FSR89 and FSR31 have structural parameters consistent with their Galactocentric distances, in the sense that tidally induced effects may have accelerated the dynamical evolution.Comment: 12 pages and 13 figs; A&A, accepted, July 9, 200

Identification of red high proper-motion objects in Tycho-2 and 2MASS catalogues using Virtual Observatory tools

Aims: With available Virtual Observatory tools, we looked for new M dwarfs in the solar neighbourhood and M giants with high tangential velocities. Methods: From an all-sky cross-match between the optical Tycho-2 and the near-infrared 2MASS catalogues, we selected objects with proper motions >50mas/yr and very red V-Ks colours. For the most interesting targets, we collected multi-wavelength photometry, constructed spectral energy distributions, estimated effective temperatures and surface gravities from fits to atmospheric models, performed time-series analysis of ASAS V-band light curves, and assigned spectral types from low-resolution spectroscopy obtained with CAFOS at the 2.2m Calar Alto telescope. Results: We got a sample of 59 bright red high proper-motion objects, including fifty red giants, four red dwarfs, and five objects reported in this work for the first time. The five new stars have magnitudes V~10.8-11.3mag, reduced proper motions midway between known dwarfs and giants, near-infrared colours typical of giants, and effective temperatures Teff~2900-3400K. From our time-series analysis, we discovered a long secondary period in Ruber 4 and an extremely long primary period in Ruber 6. With the CAFOS spectra, we confirmed the red giant nature of Ruber 7 and 8, the last of which seems to be one of the brightest metal-poor M giants ever identified.Comment: Accepted in Astronomy & Astrophysic