A detailed two-dimensional stellar population study of M32

We present Two-Dimensional Spectroscopy of the 9x12 arcsec^2 central region of M32 obtained with the 2D_FIS fibre spectrograph installed at the William Herschel Telescope. From these spectra line strength maps have been reconstructed for about 20 absorption lines, mostly belonging to the Lick system. We find good agreement with long-slit line strength profiles in the literature. In contrast with previous studies, indices were azimuthally averaged along continuum isophotes of M32. A remarkable result is that no gradients are presented in the spectral indices. So, we have fitted the mean values of each spectral index and central colours to the models of Vazdekis et al. (1996) and Worthey (1994), finding that an intermediate age (~4 Gyr) and metallicity similar to solar (Z=0.02) are the best fitted values for the innermost region of M32.Comment: accepted in MNRA

Search for radial velocity variations in eight M-dwarfs with NIRSPEC/Keck II

Context. Radial velocity (RV) measurements from near-infrared spectra have become a potentially powerful tool to search for planets around cool stars and sub-stellar objects. As part of a large survey to characterize M-dwarfs using NIRSPEC at Keck II, we obtained spectra of eight late M-dwarfs (spectral types M5.0-M8.0) during two or more observing epochs per target. These spectra were taken with intermediate spectral resolving powers (R \sim 20,000) in the J-band. Aims. We search for relative radial velocity variability in these late M-dwarfs and test the NIRSPEC capability of detecting short period brown dwarf and massive planetary companions around low-mass stars in the J-band (\approx 1.25 micron). Additionally, we reanalyzed the data of the M8-type star vB10 (one of our targets) presented in Zapatero Osorio et al. (2009), which were obtained with the same instrumentation as our data. Methods. [...] Results. For the entire M-dwarf sample, we do not find any evidence of relative RV variations induced by a short period brown dwarf or massive planetary companion. The typical RV precision of the measurements is between 180 and 300 m/s, which is sufficient to detect hot Neptunes around M-dwarfs. Also, we find that the spurious RV shift in Zapatero et al. (2009) of the star VB10 was caused by asymmetries in the instrumental profile between different observing epochs, which were not taken into account in their analysis.Comment: A&A, 7 pages, 5 figure

Single-epoch and Differential Astrometric Microlensing of Quasars

We propose and discuss a new experimental approach to measure the centroid shift induced by gravitational microlensing in the images of lensed quasars (astrometric microlensing). Our strategy is based on taking the photocenter of a region in the quasar large enough as to be insensitive to microlensing as reference to measure the centroid displacement of the continuum. In this way, single-epoch measurements of astrometric microlensing can be performed. Using numerical simulations, we show that, indeed, the centroid shift monotonically decreases as the size of the emitting region increases, and only for relatively large regions, like the broad line region (BLR), does the centroid shift become negligible. This opens interesting possibilities to study the stratification of the different emitters in the accretion disk and the BLR. We estimate the amplitude of the centroid shifts for 79 gravitationally lensed images and study more thoroughly the special cases Q2237+030 A, RXJ1131-1231 A, PG1115+080 A2 and SDSS J1004+4112 A. We propose to use spectro-astrometry to simultaneously obtain the photocenters of the continuum and of different emission line regions since, with the precision of forthcoming instruments, astrometric microlensing by $\sim 1 M_\odot$ mass microlenses may be detected in many quasar lensed images. When we consider more massive micro/millilenses, $M\gtrsim 10 M_\odot$, often proposed as the constituents of dark matter, the BLR becomes sensitive to microlensing and can no longer be used as a positional reference to measure centroid shifts. Differential microlensing between the images of a lensed quasar along several epochs should be used instead.Comment: 14 pages, 6 figures, 1 table. ApJ in pres

Fossil group origins V. The dependence of the luminosity function on the magnitude gap

In nature we observe galaxy aggregations that span a wide range of magnitude gaps between the two first-ranked galaxies of a system ($\Delta m_{12}$). There are systems with gaps close to zero (e.g., the Coma cluster), and at the other extreme of the distribution, the largest gaps are found among the so-called fossil systems. Fossil and non-fossil systems could have different galaxy populations that should be reflected in their luminosity functions. In this work we study, for the first time, the dependence of the luminosity function parameters on $\Delta m_{12}$ using data obtained by the fossil group origins (FOGO) project. We constructed a hybrid luminosity function for 102 groups and clusters at $z \le 0.25$. We stacked all the individual luminosity functions, dividing them into bins of $\Delta m_{12}$, and studied their best-fit Schechter parameters. We additionally computed a relative luminosity function, expressed as a function of the central galaxy luminosity, which boosts our capacity to detect differences, especially at the bright end. We find trends as a function of $\Delta m_{12}$ at both the bright and faint ends of the luminosity function. In particular, at the bright end, the larger the magnitude gap, the fainter the characteristic magnitude $M^\ast$. We also find differences at the faint end. In this region, the larger the gap, the flatter the faint-end slope $\alpha$. The differences found at the bright end support a dissipationless, dynamical friction-driven merging model for the growth of the central galaxy in group- and cluster-sized halos. The differences in the faint end cannot be explained by this mechanism. Other processes, such as enhanced tidal disruption due to early infall and/or prevalence of eccentric orbits, may play a role. However, a larger sample of systems with $\Delta m_{12} > 1.5$ is needed to establish the differences at the faint end.Comment: 11 pages, 10 figures, accepted for publication in A&

J-band variability of M dwarfs in the WFCAM Transit Survey

We present an analysis of the photometric variability of M dwarfs in the Wide Field Camera (WFCAM) Transit Survey. Although periodic light-curve variability in low mass stars is generally dominated by photospheric star spot activity, M dwarf variability in the J band has not been as thoroughly investigated as at visible wavelengths. Spectral type estimates for a sample of over 200 000 objects are made using spectral type-colour relations, and over 9600 dwarfs (J 0.2 mag flaring event from an M4V star in our sample.Peer reviewe

Fossil Groups Origins III. The relation between optical and X-ray luminosities

This study is part of the FOssil Groups Origin (FOGO) project which aims at carrying out a systematic and multiwavelength study of a large sample of fossil systems. Here we focus on the relation between the optical luminosity (Lopt) and X-ray luminosity (Lx). Out of a sample of 28 candidate fossil systems, we consider a sample of 12 systems whose fossil classification has been confirmed by a companion study. They are compared with the complementary sample of 16 systems whose fossil nature is not confirmed and with a subsample of 102 galaxy systems from the RASS-SDSS galaxy cluster survey. Fossil and normal systems span the same redshift range 0<z<0.5 and have the same Lx distribution. For each fossil system, the Lx in the 0.1-2.4 keV band is computed using data from the ROSAT All Sky Survey. For each fossil and normal system we homogeneously compute Lopt in the r-band within the characteristic cluster radius, using data from the SDSS DR7. We sample the Lx-Lopt relation over two orders of magnitude in Lx. Our analysis shows that fossil systems are not statistically distinguishable from the normal systems both through the 2D KS test and the fit of the Lx-Lopt relation. The optical luminosity of the galaxy system does strongly correlate with the X-ray luminosity of the hot gas component, independently of whether the system is fossil or not. We conclude that our results are consistent with the classical "merging scenario" of the brightest galaxy formed via merger/cannibalism of other group galaxies, with conservation of the optical light. We find no evidence for a peculiar state of the hot intracluster medium.Comment: A&A, 12 pages, 4 figures, 3 tables, typos corr. and paper re-numbe

Infrared radial velocities of vB 10

We present radial velocities of the M8V-type, very low-mass star vB 10 that have been obtained at four different epochs of observations between 2001 and 2008. We use high-resolution (R ~ 20,000) near-infrared (J-band) spectra taken with the NIRSPEC instrument on the Keck II telescope. Our data suggest that vB 10 shows radial velocity variability with an amplitude of ~1 km/s, a result that is consistent with the recent finding of a massive planet companion around the star by Pravdo & Shaklan (2009). More velocity measurements and a better sampling of the orbital phase are required to precisely constrain the orbital parameters and the individual masses of the pair.Comment: 4 pages, accepted for publication in A&A Letter

Two close binaries across the hydrogen-burning limit in the Praesepe open cluster

We present Keck I/OSIRIS and Keck II/NIRC2 adaptive optics imaging of two member candidates of the Praesepe stellar cluster (d=186.18$\pm$0.11 pc; 590-790 Myr), UGC J08451066+2148171 (L1.5$\pm$0.5) and UGCS J08301935$+$2003293 (no spectroscopic classification). We resolved UGCS J08451066$+$2148171 into a binary system in the near-infrared, with a $K$-band wavelength flux ratio of 0.89$\pm$0.04, a projected separation of 60.3$\pm$1.3 mas (11.2$\pm$0.7 au; 1$\sigma$). We also resolved UGCS J08301935$+$2003293 into a binary system with a flux ratio of 0.46$\pm$0.03 and a separation of 62.5$\pm$0.9 mas. Assuming zero eccentricity, we estimate minimum orbital periods of $\sim$100 years for both systems. According to theoretical evolutionary models, we derive masses in the range of 0.074-0.078 M$_{\odot}$ and 0.072-0.076 M$_{\odot}$ for the primary and secondary of UGCS J08451066$+$2148171 for an age of 700$\pm$100 Myr. In the case of UGCS J08301935$+$2003293, the primary is a low-mass star at the stellar/substellar boundary (0.070-0.078 M$_{\odot}$) while the companion candidate might be a brown dwarf (0.051-0.065 M$_{\odot}$). These are the first two binaries composed of L dwarfs in Praesepe. They are benchmark systems to derive the location of the substellar limit at the age and metallicity of Praesepe, determine the age of the cluster based on the lithium depletion boundary test, derive dynamical masses, and improve low-mass stellar and substellar evolutionary models at a well-known age and metallicity.Comment: 12 pages, 5 figures, 3 tables, accepted for publication in MNRA

Fossil Groups Origins III. Characterization of the sample and observational properties of fossil systems

(Abridged) Fossil systems are group- or cluster-sized objects whose luminosity is dominated by a very massive central galaxy. In the current cold dark matter scenario, these objects formed hierarchically at an early epoch of the Universe and then slowly evolved until present day. That is the reason why they are called {\it fossils}. We started an extensive observational program to characterize a sample of 34 fossil group candidates spanning a broad range of physical properties. Deep $r-$band images were taken for each candidate and optical spectroscopic observations were obtained for $\sim$ 1200 galaxies. This new dataset was completed with SDSS DR7 archival data to obtain robust cluster membership and global properties of each fossil group candidate. For each system, we recomputed the magnitude gaps between the two brightest galaxies ($\Delta m_{12}$) and the first and fourth ranked galaxies ($\Delta m_{14}$) within 0.5 $R_{{\rm 200}}$. We consider fossil systems those with $\Delta m_{12} \ge 2$ mag or $\Delta m_{14} \ge 2.5$ mag within the errors. We find that 15 candidates turned out to be fossil systems. Their observational properties agree with those of non-fossil systems. Both follow the same correlations, but fossils are always extreme cases. In particular, they host the brightest central galaxies and the fraction of total galaxy light enclosed in the central galaxy is larger in fossil than in non-fossil systems. Finally, we confirm the existence of genuine fossil clusters. Combining our results with others in the literature, we favor the merging scenario in which fossil systems formed due to mergers of $L^\ast$ galaxies. The large magnitude gap is a consequence of the extreme merger ratio within fossil systems and therefore it is an evolutionary effect. Moreover, we suggest that at least one candidate in our sample could represent a transitional fossil stage.Comment: 14 pages, 11 figures, accepted for publication in A&

Incidence of debris discs around FGK stars in the solar neighbourhood

Debris discs are a consequence of the planet formation process and constitute the fingerprints of planetesimal systems. Their solar system's counterparts are the asteroid and Edgeworth-Kuiper belts. The aim of this paper is to provide robust numbers for the incidence of debris discs around FGK stars in the solar neighbourhood. The full sample of 177 FGK stars with d<20 pc proposed for the DUNES survey is presented. Herschel/PACS observations at 100 and 160 micron complemented with data at 70 micron, and at 250, 350 and 500 micron SPIRE photometry, were obtained. The 123 objects observed by the DUNES collaboration were presented in a previous paper. The remaining 54 stars, shared with the DEBRIS consortium and observed by them, and the combined full sample are studied in this paper. The incidence of debris discs per spectral type is analysed and put into context together with other parameters of the sample, like metallicity, rotation and activity, and age. The subsample of 105 stars with d<15 pc containing 23 F, 33 G and 49 K stars, is complete for F stars, almost complete for G stars and contains a substantial number of K stars to draw solid conclusions on objects of this spectral type. The incidence rates of debris discs per spectral type 0.26 (6 objects with excesses out of 23 F stars), 0.21 (7 out of 33 G stars) and 0.20 (10 out of 49 K stars), the fraction for all three spectral types together being 0.22 (23 out of 105 stars). Uncertainties corresponding to a 95% confidence level are given in the text for all these numbers. The medians of the upper limits of L_dust/L_* for each spectral type are 7.8E-7 (F), 1.4E-6 (G) and 2.2E-6 (K); the lowest values being around 4.0E-7. The incidence of debris discs is similar for active (young) and inactive (old) stars. The fractional luminosity tends to drop with increasing age, as expected from collisional erosion of the debris belts.Comment: 31 pages, 15 figures, 10 tables, 2 appendice