NGC 5385, NGC 2664 and Collinder 21: three candidate Open Cluster Remnants

We present CCD UBVI photometric and medium/high resolution spectroscopic observations obtained in the field of the previously unstudied dissolving open cluster candidates NGC 5385, NGC 2664 and Collinder~21. Our analysis stands on the discussion of star counts, photometry, radial velocity distribution, and proper motions available from the Tycho~2 catalogue. All the three aggregates clearly emerge from the mean Galactic field, but, regrettably, the close scrutiny of proper motions and radial velocities reveals that we are not facing any physical group. Instead, what we are looking at are just chance alignments of a few bright unrelated stars. Our analysis casts some doubt on the Bica et al. (2001) criterion to look for Possible Open Cluster Remnants. It seems mandatory to define a better criterion to adopt for further investigations.Comment: 12 pages, 12 eps figures (7, 9 and 11 degraded in resolution), accepted for publication in Astronomy & Astrophysic

Probing galaxy evolution through the internal colour gradients, the Kormendy relations and the Photometric Plane of cluster galaxies at z~0.2

We present a detailed analysis of the photometric properties of galaxies in the cluster \A2163B at redshift z~0.2. R-, I- and K-band structural parameters, (half light radius r_e, mean surface brightness _e within r_e and Sersic index n) are derived for N~60 galaxies, and are used to study their internal colour gradients. For the first time, we use the slopes of optical-NIR Kormendy relations to study colour gradients as a function of galaxy size, and we derive the Photometric Plane at z~0.2 in the K band. Colour gradients are negligible at optical wavelengths, and are negative in the optical-NIR, implying a metallicity gradient in galaxies of ~0.2 dex per radial decade. The analysis of the Kormendy relation suggests that its slope increases from the optical to the NIR, implying that colour gradients do not vary or even do become less steep in more massive galaxies. Such a result is not simply accomodated within a monolithic collapse scenario, while it can be well understood within a hierarchical merging framework. Finally, we derive the first NIR Photometric Plane at z~0.2, accounting for both the correlations on the measurement uncertainties and the selection effects. The Photometric Plane at z~0.2 is consistent with that at z~0, with an intrinsic scatter significantly smaller than the Kormendy relation but larger than the Fundamental Plane.Comment: 18 pages, 12 figures, A&A in pres

Optical and X-ray observations of candidate isolated neutron stars in the G315.4-2.3 SNR

G315.4-2.3 is a young Galactic supernova remnant (SNR), whose identification as the remains of a Type-II supernova (SN) explosion has been debated for a long time. In particular, recent multi-wavelength observations suggest that it is the result of a Type Ia SN, based on spectroscopy of the SNR shell and the lack of a compact stellar remnant.However, two X-ray sources, one detected by Einstein and ROSAT (Source V) and the other by Chandra (Source N) have been proposed as possible isolated neutron star candidates. In both cases, no clear optical identification was available and, therefore, we performed an optical and X-ray study to determine the nature of these two sources. Based on Chandra astrometry, Source V is associated with a bright V~14 star, which had been suggested based on the less accurate ROSAT position. Similarly, from VLT archival observations, we found that Source N is associated with a relatively bright star ($V=20.14$). These likely identifications suggest that both X-ray sources cannot be isolated neutron stars.Comment: 4 pages, 4 figures, accepted for publication in MNRA

X-ray and optical observations of the unique binary system HD49798/RXJ0648.0-4418

We report the results of XMM-Newton observations of HD49798/RXJ0648.0-4418, the only known X-ray binary consisting of a hot sub-dwarf and a white dwarf. The white dwarf rotates very rapidly (P=13.2 s) and has a dynamically measured mass of 1.28+/-0.05 M_sun. Its X-ray emission consists of a strongly pulsed, soft component, well fit by a blackbody with kT~40 eV, accounting for most of the luminosity, and a fainter hard power-law component (photon index ~1.6). A luminosity of ~10^{32} erg/s is produced by accretion onto the white dwarf of the helium-rich matter from the wind of the companion, which is one of the few hot sub-dwarfs showing evidence of mass-loss. A search for optical pulsations at the South African Astronomical Observatory 1.9-m telescope gave negative results. X-rays were detected also during the white dwarf eclipse. This emission, with luminosity 2x10^{30} erg/s, can be attributed to HD 49798 and represents the first detection of a hot sub-dwarf star in the X-ray band. HD49798/RXJ0648.0-4418 is a post-common envelope binary which most likely originated from a pair of stars with masses ~8-10 M_sun. After the current He-burning phase, HD 49798 will expand and reach the Roche-lobe, causing a higher accretion rate onto the white dwarf which can reach the Chandrasekhar limit. Considering the fast spin of the white dwarf, this could lead to the formation of a millisecond pulsar. Alternatively, this system could be a Type Ia supernova progenitor with the appealing characteristic of a short time delay, being the descendent of relatively massive stars.Comment: Accepted for publication on The Astrophysical Journa

Optical (VRI) Photometry in the Field of the Galaxy Cluster AC 118 at z=0.31

We present new photometric V-, R- and I-band data for the rich galaxy cluster AC 118 at z=0.31. The new photometry covers an area of 8.6x8.6 arcmin$^2$, corresponding to 2.9$\times$2.9 Mpc$^2$ (H$_0=50$ km s$^{-1}$ Mpc$^{-1}$, q$_0=0.5$ and $\Lambda = 0$). The data have been collected for a project aimed at studying galaxy evolution through the color-magnitude relation and the fundamental plane. We provide a catalogue including all the sources (N = 1206) detected in the cluster field. The galaxy sample is complete to V=22.8 mag (N_gal=574), R=22.3 mag (N_gal=649) and I=20.8 mag (N_gal=419). We give aperture magnitudes within a fixed aperture of 4.4 arcsec and Kron magnitudes. We also give photometric redshifts for 459 sources for which additional U- and K-band photometry is available. We derive and discuss the V- and R-band luminosity functions. The catalogue, which is distributed in electronic form, is intended as a tool for studies in galaxy evolution.Comment: 15 pages, 6 figures, A&A in pres

Photometric Properties of Galaxy Population in the Cluster EIS 0048-2942 at z~0.64

~Deep photometric data in the V-, R-, I-, z- and K-bands for the cluster of galaxies EIS 0048-2942 are used to investigate the properties of the galaxy populations at z~0.64 in a field of 2.5x2.5 Mpc^2. The sample of candidate cluster members (N = 171) is selected by the photometric redshift technique and is complete up to I=22.5. Galaxies were classified as spheroids and disks according to the shape of the light profile in the I-band, as parametrized by the Sersic index. In both optical and NIR, spheroids define a sharp colour-magnitude sequence, whose slope and zero points are consistent with a high formation redshift (z_f > 2). The disk population occupies a different region in the colour-magnitude diagram, having bluer colours with respect to the red sequence. Interestingly, we find some level of mixing between the properties of the two classes: some disks lie on the colour-magnitude sequence or are redder, while some spheroids turn out to be bluer. The spatial distribution of cluster galaxies show a clumpy structure, with a main over-density of radius ~0.5 Mpc, and at least two other clumps distant ~1 Mpc from the center. The various sub-structures are mostly populated by the red galaxies, while the blue population has an almost uniform distribution. The fraction of blue galaxies in EIS 0048-2942 is f_B=0.11 +/-0.07. This is much lower than what expected on the basis of the Butcher-Oemler effect at lower redshifts.Comment: 22 pages, 12 figures. A&A in pres

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

Bayesian analysis to identify new star candidates in nearby young stellar kinematic groups

We present a new method based on a Bayesian analysis to identify new members of nearby young kinematic groups. The analysis minimally takes into account the position, proper motion, magnitude and color of a star, but other observables can be readily added (e.g. radial velocity, distance). We use this method to find new young low-mass stars in the \beta Pictoris (\beta PMG) and AB Doradus (ABDMG) moving groups and in the TW Hydrae (TWA), Tucana-Horologium (THA), Columba, Carina and Argus associations. Starting from a sample of 758 mid-KM (K5V-M5V) stars showing youth indicators such as H\alpha\ and X-ray emission, our analysis yields 215 new highly probable low-mass members of the kinematic groups analyzed. One is in TWA, 37 in \beta PMG, 17 in THA, 20 in Columba, 6 in Carina, 50 in Argus, 33 in ABDMG, and the remaining 51 candidates are likely young but have an ambiguous membership to more than one association. The false alarm rate for new candidates is estimated to be 5% for \beta PMG and TWA, 10% for THA, Columba, Carina and Argus, and 14% for ABDMG. Our analysis confirms the membership of 58 stars proposed in the literature. Firm membership confirmation of our new candidates will require measurement of their radial velocity (predicted by our analysis), parallax and lithium 6708 {\AA} equivalent width. We have initiated these follow-up observations for a number of candidates and we have identified two stars (2MASSJ0111+1526, 2MASSJ0524-1601) as very strong candidate members of the \beta PMG and one strong candidate member (2MASSJ0533-5117) of the THA; these three stars have radial velocity measurements confirming their membership and lithium detections consistent with young age. Finally, we proposed that six stars should be considered as new bona fide members of \beta PMG and ABDMG, one of which being first identified in this work, the others being known candidates from the literature.Comment: Accepted for publication in Ap

The short-duration GRB 050724 host galaxy in the context of the long-duration GRB hosts

We report optical and near-infrared broad band observations of the short-duration GRB 050724 host galaxy, used to construct its spectral energy distribution (SED). Unlike the hosts of long-duration gamma-ray bursts (GRBs), which show younger stellar populations, the SED of the GRB 050724 host galaxy is optimally fitted with a synthetic elliptical galaxy template based on an evolved stellar population (age ~2.6 Gyr). The SED of the host is difficult to reproduce with non-evolving metallicity templates. In contrast, if the short GRB host galaxy metallicity enrichment is considered, the synthetic templates fit the observed SED satisfactorily. The internal host extinction is low (A_v \~< 0.4 mag) so it cannot explain the faintness of the afterglow. This short GRB host galaxy is more massive (~5x10^10 Mo) and luminous (~1.1 L*) than most of the long-duration GRB hosts. A statistical comparison based on the ages of short- and long-duration GRB host galaxies strongly suggests that short-duration GRB hosts contain, on average, older progenitors. These findings support a different origin for short- and long-duration GRBs.Comment: A&A in pres

The intermediate-redshift galaxy cluster CL 0048-2942. Stellar populations

We present a detailed study of the cluster CL 0048-2942, located at z~0.64, based on a photometric and spectroscopic catalogue of 54 galaxies in a 5 x 5 square arcmin region centred in that cluster. Of these, 23 galaxies were found to belong to the cluster. Based on this sample, the line-of-sight velocity dispersion of the cluster is approximately 680 +- 140 km/s. We have performed stellar population synthesis in the cluster members as well as in the field galaxies of the sample and found that there are population gradients in the cluster with central galaxies hosting mainly intermediate/old populations whereas galaxies in the cluster outskirts show clearly an increase of younger populations, meaning that star formation is predominantly taking place in the outer regions of the cluster. In a general way, field galaxies seem to host less evolved stellar populations than cluster members. In fact, in terms of ages, young supergiant stars dominate the spectra of field galaxies whereas cluster galaxies display a dominant number of old and intermediate age stars. Following the work of other authors (e.g. Dressler et al. 1999) we have estimated the percentage of K+A galaxies in our sample and found around 13% in the cluster and 10% in the field. These values were estimated through means of a new method, based on stellar population synthesis results, that takes into account all possible absorption features in the spectrum and thus makes optimal use of the data.Comment: Accepted by Astronomy & Astrophysics. 24 pages, 10 figures, 10 tables (figures 3, 4, 5 and tables 1, 3, 4, 5, 6, 7, 8 will be available in electronic format only in the A&A published version