Search

We report the results from a photometric reverberation mapping campaign carried out with the C18 telescope at the Wise Observatory from 2019 to 2020, targeting the active galactic nucleus (AGN) MCG 08-11-011. The monitoring was conducted on a daily basis with specially designed narrow-band filters, spanning from optical to near-infrared wavelengths ($\sim4000$ to $8000${\AA}) and avoiding prominent broad emission lines. We aim to measure inter-band continuum time lags, determine the size-wavelength relation, and estimate the host-subtracted AGN luminosity for this system. We used the point-spread function photometry to extract the continuum light curves and measure the inter-band time lags using several methods, including the interpolated cross-correlation function, the z-transformed discrete correlation function, a von Neumann estimator, JAVELIN (in spectroscopic and photometric mode), MICA, and a multivariate correlation function. We find wavelength-dependent lags, $\tau(\lambda)$, up to $\sim$7 days between the multiband light curves of MCG 08-11-011. The observed lags are larger than predictions based on standard thin-disk theory by a factor of $\sim3-7$. We discern a significantly steeper ($\tau \propto \lambda^{4.74}$) size-wavelength relation than the $\tau \propto \lambda^{4/3}$ expected for a geometrically thin and optically thick accretion disk, which may result from the contribution of diffuse continuum emission to the flux. These results are similar to those found by previous continuum reverberation mapping campaigns., Comment: arXiv admin note: text overlap with arXiv:2111.07385

We present the results of an intensive six-month optical continuum reverberation mapping campaign of the Seyfert 1 galaxy PG 2130+099 at redshift z=0.063. The ground-based photometric monitoring was conducted on a daily basis with the robotic 46cm telescope of the Wise observatory located in Israel. Specially designed narrowband filters were used to observe the central engine of the active galactic nucleus (AGN), avoiding line contamination from the broad-line region (BLR). We aim to measure inter-band continuum time lags across the optical range and determine the size-wavelength relation for this system. We used two methods, the traditional point-spread function (PSF) photometry and the recently developed proper image subtraction technique, to independently perform the extraction of the continuum light curves. The inter-band time lags are measured with several methods, including the interpolated cross-correlation function, the z-transformed discrete correlation function, a von Neumann estimator, JAVELIN (in spectroscopic mode), and MICA. PG 2130+099 displays correlated variability across the optical range, and we successfully detect significant time lags of up to 3 days between the multiband light curves. Using the flux variation gradient (FVG) method, we determined the AGN's host-galaxy-subtracted rest-frame 5100{\AA} luminosity at the time of our monitoring campaign with an uncertainty of 18%. While a continuum reprocessing model can fit the data reasonably well, our derived disk sizes are a factor of 2-6 larger than the theoretical disk sizes predicted from the AGN luminosity estimate of PG 2130+099. This result is in agreement with previous studies of AGN/quasars and suggests that the standard Shakura-Sunyaev disk theory has limitations in describing AGN accretion disks.

Close double degenerate binaries are one of the favoured progenitor channels for type Ia supernovae, but it is unclear how many suitable systems there are in the Galaxy. We report results of a large radial velocity survey for double degenerate (DD) binaries using the UVES spectrograph at the ESO VLT (ESO SN Ia Progenitor surveY - SPY). Exposures taken at different epochs are checked for radial velocity shifts indicating close binary systems. We observed 689 targets classified as DA (displaying hydrogen-rich atmospheres), of which 46 turned out to possess a cool companion. We measured radial velocities (RV) of the remaining 643 DA white dwarfs. We managed to secure observations at two or more epochs for 625 targets, supplemented by eleven objects meeting our selection criteria from literature. The data reduction and analysis methods applied to the survey data are described in detail. The sample contains 39 double degenerate binaries, only four of which were previously known. 20 are double-lined systems, in which features from both components are visible, the other 19 are single-lined binaries. We provide absolute RVs transformed to the heliocentric system suitable for kinematic studies. Our sample is large enough to sub-divide by mass: 16 out of 44 low mass targets (<= 0.45 Msun) are detected as DDs, while just 23 of the remaining 567 with multiple spectra and mass >0.45 Msun are double. Although the detected fraction amongst the low mass objects (36.4 +/- 7.3%) is significantly higher than for the higher-mass, carbon/oxygen-core dominated part of the sample (3.9 +/- 0.8%), it is lower than the detection efficiency based upon companion star masses >= 0.05 Msun. This suggests either companion stars of mass < 0.05 Msun, or that some of the low mass white dwarfs are single., Comment: Accepted for publication in A&A. 59 pages. Think before you print: large tables in the appendix

The upcoming NASA Transiting Exoplanet Survey Satellite (TESS) will obtain space-based uninterrupted light curves for a large sample of bright white dwarfs distributed across the entire sky, providing a very rich resource for asteroseismological studies and the search for transits from planetary debris. We have compiled an all-sky catalogue of ultraviolet, optical, and infrared photometry as well as proper motions, which we propose as an essential tool for the preliminary identification and characterisation of potential targets. We present data for 1864 known white dwarfs and 305 high-probability white dwarf candidates brighter than 17 mag. We describe the spectroscopic follow-up of 135 stars, of which 82 are white dwarfs and 25 are hot subdwarfs. The new confirmed stars include six pulsating white dwarf candidates (ZZ Cetis), and nine white dwarf binaries with a cool main-sequence companion. We identify one star with a spectroscopic distance of only 25 pc from the Sun. Around the time TESS is launched, we foresee that all white dwarfs in this sample will have trigonometric parallaxes measured by the ESA Gaia mission next year., Comment: Accepted for publication on MNRAS. 21 pages, 8 tables. Machine readable tables, which will be made also available through VizieR, are also included

We investigated the prospects for systematic searches of white dwarfs at low Galactic latitudes, using the VLT Survey Telescope (VST) H$\alpha$ Photometric Survey of the Galactic plane and Bulge (VPHAS+). We targeted 17 white dwarf candidates along sightlines of known open clusters, aiming to identify potential cluster members. We confirmed all the 17 white dwarf candidates from blue/optical spectroscopy, and we suggest five of them to be likely cluster members. We estimated progenitor ages and masses for the candidate cluster members, and compared our findings to those for other cluster white dwarfs. A white dwarf in NGC 3532 is the most massive known cluster member (1.13 M$_{\odot}$), likely with an oxygen-neon core, for which we estimate an $8.8_{-4.3}^{+1.2}$ M$_{\odot}$ progenitor, close to the mass-divide between white dwarf and neutron star progenitors. A cluster member in Ruprecht 131 is a magnetic white dwarf, whose progenitor mass exceeded 2-3 M$_{\odot}$. We stress that wider searches, and improved cluster distances and ages derived from data of the ESA Gaia mission, will advance the understanding of the mass-loss processes for low- to intermediate-mass stars., Comment: 17 pages, 11 figures, 5 tables. Accepted for publication in MNRAS

We analyze a time series of optical spectra of SN~2014J from almost two weeks prior to maximum to nearly four months after maximum. We perform our analysis using the SYNOW code, which is well suited to track the distribution of the ions with velocity in the ejecta. We show that almost all of the spectral features during the entire epoch can be identified with permitted transitions of the common ions found in normal SNe Ia in agreement with previous studies. We show that 2014J is a relatively normal SN Ia. At early times the spectral features are dominated by Si II, S II, Mg II, and Ca II. These ions persist to maximum light with the appearance of Na I and Mg I. At later times iron-group elements also appear, as expected in the stratified abundance model of the formation of normal type Ia SNe. We do not find significant spectroscopic evidence for oxygen, until 100 days after maximum light. The +100 day identification of oxygen is tentative, and would imply significant mixing of unburned or only slight processed elements down to a velocity of 6,000 km/s. Our results are in relatively good agreement with other analyses in the IR. We briefly compare SN 2011fe to SN 2014J and conclude that the differences could be due to different central densities at ignition or differences in the C/O ratio of the progenitors., Comment: 13 pages, 6 figures, 4 tables, MNRAS, in press

We present the intensive spectroscopic follow up of the type Ia supernova (SN Ia) 2014J in the starburst galaxy M82. Twenty-seven optical spectra have been acquired from January 22nd to September 1st 2014 with the Isaac Newton (INT) and William Herschel (WHT) Telescopes. After correcting the observations for the recession velocity of M82 and for Milky Way and host galaxy extinction, we measured expansion velocities from spectral line blueshifts and pseudo-equivalent width of the strongest features in the spectra, which gives an idea on how elements are distributed within the ejecta. We position SN 2014J in the Benetti (2005), Branch et al. (2006) and Wang et al. (2009) diagrams. These diagrams are based on properties of the Si II features and provide dynamical and chemical information about the SN ejecta. The nearby SN 2011fe, which showed little evidence for reddening in its host galaxy, is shown as a reference for comparisons. SN 2014J is a border-line object between the Core-normal (CN) and Broad-line (BL) groups, which corresponds to an intermediate position between Low Velocity Gradient (LVG) and High Velocity Gradient (HVG) objects. SN 2014J follows the R(Si II)-\Delta m15 correlation, which confirms its classification as a relatively normal SN Ia. Our description of the SN Ia in terms of the evolution of the pseudo-equivalent width of various ions as well as the position in the various diagrams put this specific SN Ia into the overall sample of SN Ia., Comment: 14 pages, 10 Figures, 4 Tables. Accepted in MNRAS

This paper presents the simultaneous Bayesian analysis of the radial velocity and activity indices for the K5Ve active star BD+20 1790, that was proposed to host a close-in massive planet by Hern\'an-Obispo et al. (2010). The Bayesian analysis supports the hypothesis of a planetary companion. We present a new orbital solution after removing the two main contributions to the stellar jitter, one that varies with the photometric period and another that varies with the synodic period of the star+planet system. We use a new method to determine these jitter components, considering them as a second and third signals in the system. A discussion on possible magnetic star-planet-interaction is included, based on the Bayesian analysis of the activity indices. We propose two possible sources for flare events of this star: one related to the geometry of the system and the relative movement of the star+planet, and a second one purely stochastic source that is related to the evolution of stellar active regions. Also, we observe for the first time the magnetic field of the star, from spectropolarimetric data., Comment: 17 pages, 18 figures, accepted for publication at A\&A (!2 January 2015)

We report the discovery of WTS-1b, the first extrasolar planet found by the WFCAM Transit Survey, which began observations at the 3.8-m United Kingdom Infrared Telescope (UKIRT) in August 2007. Light curves comprising almost 1200 epochs with a photometric precision of better than 1 per cent to J ~ 16 were constructed for ~60000 stars and searched for periodic transit signals. For one of the most promising transiting candidates, high-resolution spectra taken at the Hobby-Eberly Telescope (HET) allowed us to estimate the spectroscopic parameters of the host star, a late-F main sequence dwarf (V=16.13) with possibly slightly subsolar metallicity, and to measure its radial velocity variations. The combined analysis of the light curves and spectroscopic data resulted in an orbital period of the substellar companion of 3.35 days, a planetary mass of 4.01 +- 0.35 Mj and a planetary radius of 1.49+0.16-0.18 Rj. WTS-1b has one of the largest radius anomalies among the known hot Jupiters in the mass range 3-5 Mj. The high irradiation from the host star ranks the planet in the pM class., Comment: 16 pages, 10 figures

We present high time resolution SDSS-$g'$ and SDSS-$z'$ light curves of the primary eclipse in SDSS J141126.20+200911.1, together with time-resolved X-Shooter spectroscopy and near-infrared $JHK_{s}$ photometry. Our observations confirm the substellar nature of the companion, making SDSS J141126.20+200911.1 the first eclipsing white dwarf/brown dwarf binary known. We measure a (white dwarf model dependent) mass and radius for the brown dwarf companion of $M_{2} = 0.050 \pm 0.002$ $M_{\odot}$ and $R_{2} = 0.072 \pm 0.004$ $M_{\odot}$, respectively. The lack of a robust detection of the companion light in the $z'$-band eclipse constrains the spectral type of the companion to be later than L5. Comparing the NIR photometry to the expected white dwarf flux reveals a clear $K_s$-band excess, suggesting a spectral type in the range L7-T1. The radius measurement is consistent with the predictions of evolutionary models, and suggests a system age in excess of three Gyr. The low companion mass is inconsistent with the inferred spectral type of L7-T1, instead predicting a spectral type nearer T5. This indicates that irradiation of the companion in SDSS J1411 could be causing a significant temperature increase, at least on one hemisphere., Comment: 11 pages, 10 figures, accepted for publication in MNRAS

This is the product of a Gaia GREAT workshop on "Gaia and the end states of stellar evolution" held at the University of Leicester in April 2011.

The star 1SWASP J162842.31+101416.7 (WASP 1628+10) is one of several EL CVn-type stars recently identified using the WASP database, i.e., an eclipsing binary star in which an A-type dwarf star (WASP 1628+10A) eclipses the remnant of a disrupted red giant star (WASP1628+10B). We have measured the masses, radii and luminosities of the stars in WASP 1628+10 using photometry obtained in three bands (u', g', r') with the Ultracam instrument and medium-resolution spectroscopy. The properties of the remnant are well-matched by models for stars in a rarely-observed state evolving to higher effective temperatures at nearly constant luminosity prior to becoming a very low-mass white dwarf composed almost entirely of helium, i.e., we confirm that WASP 1628+10B is a pre-He-WD. WASP 1628+10A appears to be a normal A2V star with a mass of $1.36 \pm 0.05 M_{\odot}$. By fitting models to the spectrum of this star around the H$\gamma$ line we find that it has an effective temperature T$_{\rm eff,A} = 7500 \pm 200$K and a metallicity [Fe/H]$ = -0.3 \pm 0.3$. The mass of WASP 1628+10B is only $0.135 \pm 0.02M_{\odot}$. The effective temperature of this pre-He-WD is approximately 9200K. The Ultracam photometry of WASP 1628+10 shows variability at several frequencies around 40 cycles per day, which is typical for $\delta$ Sct-type pulsations often observed in early A-type stars like WASP 1628+10A. We also observe frequencies near 114 cycles/day and 129 cycles/day, much higher than the frequencies normally seen in $\delta$ Sct stars. Additional photometry through the primary eclipse will be required to confirm that these higher frequencies are due to pulsations in WASP 1628+10B. If confirmed, this would be only the second known example of a pre-He-WD showing high-frequency pulsations., Comment: 9 pages, 7 figures. Accepted for publication in MNRAS. Added missing bibliography

The UKIDSS Galactic Plane Survey (GPS) began in 2005 as a 7 year effort to survey ~1800 square degrees of the northern Galactic plane in the J, H, and K passbands. The survey included a second epoch of K band data, with a baseline of 2 to 8 years, for the purpose of investigating variability and measuring proper motions. We have calculated proper motions for 167 Million sources in a 900 square degree area located at l > 60 degrees in order to search for new high proper motion objects. Visual inspection has verified 617 high proper motion sources (> 200 mas/yr) down to K=17, of which 153 are new discoveries. Among these we have a new spectroscopically confirmed T5 dwarf, an additional T dwarf with estimated type T6, 13 new L dwarf candidates, and two new common proper motion systems containing ultracool dwarf candidates. We provide improved proper motions for an additional 12 high proper motion stars that were independently discovered in the WISE dataset during the course of this investigation., Comment: Accepted for publication in the MNRAS main journal

Context. White dwarfs can be used to study the structure and evolution of the Galaxy by analysing their luminosity function and initial mass function. Among them, the very cool white dwarfs provide the information for the early ages of each population. Because white dwarfs are intrinsically faint only the nearby (about 20 pc) sample is reasonably complete. The Gaia space mission will drastically increase the sample of known white dwarfs through its 5-6 years survey of the whole sky up to magnitude V = 20-25. Aims. We provide a characterisation of Gaia photometry for white dwarfs to better prepare for the analysis of the scientific output of the mission. Transformations between some of the most common photometric systems and Gaia passbands are derived. We also give estimates of the number of white dwarfs of the different galactic populations that will be observed. Methods. Using synthetic spectral energy distributions and the most recent Gaia transmission curves, we computed colours of three different types of white dwarfs (pure hydrogen, pure helium, and mixed composition with H/He= 0.1). With these colours we derived transformations to other common photometric systems (Johnson-Cousins, Sloan Digital Sky Survey, and 2MASS). We also present numbers of white dwarfs predicted to be observed by Gaia. Results. We provide relationships and colour-colour diagrams among different photometric systems to allow the prediction and/or study of the Gaia white dwarf colours. We also include estimates of the number of sources expected in every galactic population and with a maximum parallax error. Gaia will increase the sample of known white dwarfs tenfold to about 200 000. Gaia will be able to observe thousands of very cool white dwarfs for the first time, which will greatly improve our understanding of these stars and early phases of star formation in our Galaxy., Comment: 17 pages, 39 figures

We report the discovery of WTS-2 b, an unusually close-in 1.02-day hot Jupiter (Mp=1.12MJ, Rp=1.363RJ) orbiting a K2V star, which has a possible gravitationally-bound M-dwarf companion at 0.6 arcsec separation contributing ~20 percent of the total flux in the observed J-band light curve. The planet is only 1.5 times the separation from its host star at which it would be destroyed by Roche lobe overflow, and has a predicted remaining lifetime of just ~40 Myr, assuming a tidal dissipation quality factor of Q'*=10^6. Q'* is a key factor in determining how frictional processes within a host star affect the orbital evolution of its companion giant planets, but it is currently poorly constrained by observations. We calculate that the orbital decay of WTS-2 b would correspond to a shift in its transit arrival time of T_shift~17 seconds after 15 years assuming Q'*=10^6. A shift less than this would place a direct observational constraint on the lower limit of Q'* in this system. We also report a correction to the previously published expected T_shift for WASP-18 b, finding that T_shift=356 seconds after 10 years for Q'*=10^6, which is much larger than the estimated 28 seconds quoted in WASP-18 b discovery paper. We attempted to constrain Q'* via a study of the entire population of known transiting hot Jupiters, but our results were inconclusive, requiring a more detailed treatment of transit survey sensitivities at long periods. We conclude that the most informative and straight-forward constraints on Q'* will be obtained by direct observational measurements of the shift in transit arrival times in individual hot Jupiter systems. We show that this is achievable across the mass spectrum of exoplanet host stars within a decade, and will directly probe the effects of stellar interior structure on tidal dissipation., Comment: 21 pages, 16 figures, 6 tables, accepted for publication in MNRAS

The UKIDSS Large Area Survey (LAS) began in 2005, with the start of the UKIDSS program as a 7 year effort to survey roughly 4000 deg$^2$ at high galactic latitudes in Y, J, H and K bands. The survey also included a significant quantity of 2-epoch J band observations, with an epoch baseline greater than 2 years to calculate proper motions. We present a near infrared proper motion catalogue for the 1500 deg$^2$ of the 2 epoch LAS data, which includes 135,625 stellar sources and a further 88,324 with ambiguous morphological classifications, all with motions detected above the 5$\sigma$ level. We developed a custom proper motion pipeline which we describe here. Our catalogue agrees well with the proper motion data supplied for a 300 deg$^2$ subset in the current WFCAM Science Archive (WSA) tenth data release (DR10) catalogue, and in various optical catalogues, but it benefits from a larger matching radius and hence a larger upper proper motion detection limit. We provide absolute proper motions, using LAS galaxies for the relative to absolute correction. By using local 2nd order polynomial transformations, as opposed to linear transformations in the WSA, we correct better for any local distortions in the focal plane, not including the radial distortion that is removed by the UKIDSS pipeline. We present the results of proper motion searches for new brown dwarfs and white dwarfs. We discuss 41 sources in the WSA DR10 overlap with our catalogue with proper motions $>$300 $mas yr^{-1}$, several of which are new detections. We present 15 new candidate ultra-cool dwarf binary systems., Comment: 27 pages, 15 figures, and 8 tables. Public catalogue available at http://star.herts.ac.uk/~lsmith/

Red subdwarfs in binary systems are crucial for both model calibration and spectral classification. We search for red subdwarfs in binary systems from a sample of high proper motion objects with Sloan digital Sky Survey spectroscopy. We present here discoveries from this search, as well as highlight several additional objects of interest. We find 30 red subdwarfs in wide binary systems including: two with spectral type of esdM5.5, 6 companions to white dwarfs and 3 carbon enhanced red subdwarfs with normal red subdwarf companions. 15 red subdwarfs in our sample are partially resolved close binary systems. With this binary sample, we estimate the low limit of the red subdwarf binary fraction of ~ 10%. We find that the binary fraction goes down with decreasing masses and metallicities of red subdwarfs. A spectroscopic esdK7 subdwarf + white dwarf binary candidate is also reported. 30 new M subdwarfs have spectral type of >= M6 in our sample. We also derive relationships between spectral types and absolute magnitudes in the optical and near-infrared for M and L subdwarfs, and we present an M subdwarf sample with measured U, V, W space velocities., Comment: 26 pages, 19 figures

The evolution of white dwarfs is a simple gravothermal process. This process can be tested in two ways, through the luminosity function of these stars and through the secular variation of the period of pulsation of those stars that are variable. Here we show how the mass of the axion can be constrained using the white dwarf luminosity function., Comment: Proceedings of the 8th Patras Workshop on Axions, WIMPs and WISPs

We present an analysis of the photometric variability of M dwarfs in the WFCAM Transit Survey. Although periodic lightcurve 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<17) with spectral types later than K7 were found. The light curves of the late-type sample are searched for periodicity using a Lomb-Scargle periodogram analysis. A total of 68 periodic variable M dwarfs are found in the sample with periods ranging from 0.16 days to 90.33 days, with amplitudes in the range of ~0.009 to ~0.115 in the J band. We simulate active M dwarfs with a range of latitude-independent spot coverages and estimate a periodically variable fractions of 1-3 per cent for stars where spots cover more than 10 per cent of the star's surface. Our simulated spot distributions indicate that operating in the J band, where spot contrast ratios are minimised, enables variability in only the most active of stars to be detected. These findings affirm the benefits of using the $J$ band for planetary transit searches compared to visible bands. We also serendipitously find a \Delta J>0.2 mag flaring event from an M4V star in our sample., Comment: 18 pages, 18 figures, 2 tables, accepted in MNRAS

Recent surveys have allowed to derive the white dwarf mass distribution with reasonable accuracy. This distribution shows a noticeable degree of structure that it is often attributed to the evolution of close binaries in general, and to mergers in particular. To analyze if the origin of this structure can be attributed to the merger of double white dwarfs, we have used a simplified population synthesis model that retains the essential processes of formation of double degenerate binaries. Special care has been taken to avoid artifacts introduced by discontinuities in the distribution functions. Our result is that these structures are not probably due to mergers, but they can provide a deep insight on the evolution of close binary systems., Comment: Proceedings of the 18th European White Dwarf Workshop

We report the discovery of WTS-1b, the first extrasolar planet found by the WFCAM Transit Survey, which began observations at the 3.8-m United Kingdom Infrared Telescope. Light curves comprising almost 1200 epochs with a photometric precision of better than 1 per cent to J=16 were constructed for 60000 stars and searched for periodic transit signals. For one of the most promising transiting candidates, high-resolution spectra taken at the Hobby-Eberly Telescope allowed us to estimate the spectroscopic parameters of the host star, a late-F main sequence dwarf (V=16.13) with possibly slightly subsolar metallicity, and to measure its radial velocity variations. The combined analysis of the light curves and spectroscopic data resulted in an orbital period of the substellar companion of 3.35 days, a planetary mass of 4.01+-0.35 Mj and a planetary radius of 1.49+-0.17 Rj. WTS-1b has one of the largest radius anomalies among the known hot Jupiters in the mass range 3-5 Mj., Comment: 15 pages, 10 figures, 9 tables, accepted for publication in MNRAS

We report the identification of LSR J0745+2627 in the United Kingdom InfraRed Telescope Infrared Deep Sky Survey (UKIDSS) Large Area Survey (LAS) as a cool white dwarf with kinematics and age compatible with the thick-disk/halo population. LSR J0745+2627 has a high proper motion (890 mas/yr) and a high reduced proper motion value in the J band (H_J=21.87). We show how the infrared-reduced proper motion diagram is useful for selecting a sample of cool white dwarfs with low contamination. LSR J0745+2627 is also detected in the Sloan Digital Sky Survey (SDSS) and the Wide-field Infrared Survey Explorer (WISE). We have spectroscopically confirmed this object as a cool white dwarf using X-Shooter on the Very Large Telescope. A detailed analysis of its spectral energy distribution reveals that its atmosphere is compatible with a pure-H composition model with an effective temperature of 3880+-90 K. This object is the brightest pure-H ultracool white dwarf (Teff<4000 K) ever identified. We have constrained the distance (24-45 pc), space velocities and age considering different surface gravities. The results obtained suggest that LSR J0745+2627 belongs to the thick-disk/halo population and is also one of the closest ultracool white dwarfs., Comment: 5 pages, 7 figures, accepted for publication in A&A Letters

White dwarfs are almost completely degenerate objects that cannot obtain energy from thermonuclear sources, so their evolution is just a gravothermal cooling process. Recent improvements in the accuracy and precision of the luminosity function and in pulsational data of variable white dwarfs suggest that they are cooling faster than expected from conventional theory. In this contribution we show that the inclusion of an additional cooling term due to axions able to interact with electrons with a coupling constant g_ae ~(2-7)x10^{-13} allows to fit better the observations., Comment: 4 pages, 1 figure. Proceedings for the 7th Patras Workshop on Axions, WIMPs and WISPs (PATRAS 2011), Mykonos, Greece, June 27 - July 1, 2011

We have searched the WISE first data release for widely separated (<10,000AU) late T dwarf companions to Hipparcos and Gliese stars. We have discovered a new binary system containing a K-band suppressed T8p dwarf WISEP J1423+0116 and the mildly metal poor ([Fe/H]=-0.38+-0.06) primary BD+01 2920 (Hip 70319), a G1 dwarf at a distance of 17.2pc. This new benchmark has Teff=680+-55K and a mass of 20-50 Mjup. Its spectral properties are well modelled except for known discrepancies in the Y and K bands. Based on the well determined metallicity of its companion, the properties of BD+01 2920B imply that the currently known T dwarfs are dominated by young low-mass objects. We also present an accurate proper motion for the T8.5 dwarf WISEP J075003.84+272544.8., Comment: MNRAS, accepted 2012 January 12

Stellar ages are extremely difficult to determine and often subject to large uncertainties, especially for field low-mass stars. We plan to carry out a calibration of the decrease in high-energy emissions of low-mass GKM stars with time, and therefore precise age determination is a key ingredient. The overall goal of our research is to study the time evolution of these high-energy emissions as an essential input to studing exoplanetary atmospheres. We propose to determine stellar ages with a methodology based on wide binaries. We are interested in systems composed of a low-mass star and a white dwarf (WD), where the latter serves as a stellar chronometer for the system. We aim at obtaining reliable ages for a sample of late-type stars older than 1 Gyr. We selected a sample of wide binaries composed by a DA type WD and a GKM companion. High signal-to-noise, low-resolution spectroscopic observations were obtained for most of the WD members of the sample. Atmospheric parameters were determined by fitting the spectroscopic data to appropiate WD models. The total ages of the systems were derived by using cooling sequences, an initial-final mass relationship and evolutionary tracks, to account for the progenitor life. The spectroscopic observations have allowed us to determine ages for the binary systems using WDs as cosmochronometers. We obtained reliable ages for 27 stars between 1 and 5 Gyr, which is a range where age determination becomes difficult for field objects., Comment: 10 pages, 8 figures, Accepted by A\&A 2011 March 30

White dwarfs are almost completely degenerate objects that cannot obtain energy from the thermonuclear sources and their evolution is just a gravothermal process of cooling. The simplicity of these objects, the fact that the physical inputs necessary to understand them are well identified, although not always well understood, and the impressive observational background about white dwarfs make them the most well studied Galactic population. These characteristics allow to use them as laboratories to test new ideas of physics. In this contribution we discuss the robustness of the method and its application to the axion case., Comment: 4 pages, 1 figure, to appear in the Proceedings for the 6th Patras meeting on Axions, WIMPs and WISPs

We present results of a survey of post-asymptotic giant branch stars (post-AGBs) at high galactic latitude. To date, few post-AGB stars are known throughout the Galaxy and the number of known members of the older populations like the galactic halo is even smaller. This study looks at the number of post-AGB stars which are produced using different synthetic population methods and compare the results with observations. The resulting synthetic populations are compared to observational results from a complete and studied subsample from the photographic Palomar-Green (PG) survey (with high resolution spectroscopic follow-up for post-AGB candidates) and the SDSS spectroscopic database. The results show only two candidate post-AGB stars in a complete subsample of the PG survey spanning 4200 square degrees and one in the SDSS database. We discuss and explore any observational biases which may cause the result. If found to be truely representative of the halo population, one can expect the majority of Population II stars to fail to ascend the AGB and evolve through other evolutionary channels such as the extended horizontal branch., Comment: 6 pages, 2 figures, 1 table, Proceedings to the 17th European White Dwarf Workshop, T\"ubingen

A number of so-called ultra-cool white dwarfs have been detected in different surveys so far. However, based on anecdotal evidence it is believed that most or all of these ultra-cool white dwarfs are low-mass products of binary evolution and thus not representative for the oldest white dwarfs. Their low mass causes relatively high luminosity making them the first cool white dwarfs detected in relatively shallow surveys. Deeper observations are needed for the oldest, high mass white dwarfs with the longest cooling times. We report results of an ongoing project that combines deep IR and optical data. This combination plus proper motion information will allow an unambiguous identification of very cool white dwarfs, since the spectral energy distributions are very different from other types of stellar objects. The atmospheric parameters that can be derived from the spectral energy distributions together with the proper motions inferred from the IR data can be used to construct the white dwarf luminosity functions for the thick disc and halo populations. From these we will be able to test the early star formation history and initial mass function of the first stellar populations., Comment: 6 pages, 5 figures, to appear in AIP Conference Proceedings for the 17th European White Dwarf Workshop

We present the discovery of the first T dwarf + white dwarf binary system LSPM 1459+0857AB, confirmed through common proper motion and spectroscopy. The white dwarf is a high proper motion object from the LSPM catalogue that we confirm spectroscopically to be a relatively cool (Teff=5535+\-45K) and magnetic (B~2MG) hydrogen-rich white dwarf, with an age of at least 4.8Gyrs. The T dwarf is a recent discovery from the UKIRT Infrared Deep Sky Survey (ULAS 1459+0857), and has a spectral type of T4.5+\-0.5 and a distance in the range 43-69pc. With an age constraint (inferred from the white dwarf) of >4.8Gyrs we estimate Teff=1200-1500K and logg=5.4-5.5 for ULAS 1459+0857, making it a benchmark T dwarf with well constrained surface gravity. We also compare the T dwarf spectra with the latest LYON group atmospheric model predictions, which despite some shortcomings are in general agreement with the observed properties of ULAS 1459+0857. The separation of the binary components (16,500-26,500AU, or 365 arcseconds on the sky) is consistent with an evolved version of the more common brown dwarf + main-sequence binary systems now known, and although the system has a wide separation, it is shown to be statistically robust as a non spurious association. The observed colours of the T dwarf show that it is relatively bright in the z band compared to other T dwarfs of similar type, and further investigation is warranted to explore the possibility that this could be a more generic indicator of older T dwarfs. Future observations of this binary system will provide even stronger constraints on the T dwarf properties, and additional systems will combine to give a more comprehensively robust test of the model atmospheres in this temperature regime., Comment: 14 pages, 11 figures, MNRAS accepted

The early evolution of Earth's atmosphere and the origin of life took place at a time when physical conditions at the Earth where radically different from its present state. The radiative input from the Sun was much enhanced in the high-energy spectral domain, and in order to model early planetary atmospheres in detail, a knowledge of the solar radiative input is needed. We present an investigation of the atmospheric parameters, state of evolution and high-energy fluxes of the nearby star kap^1 Cet, previously thought to have properties resembling those of the early Sun. Atmospheric parameters were derived from the excitation/ionization equilibrium of Fe I and Fe II, profile fitting of Halpha and the spectral energy distribution. The UV irradiance was derived from FUSE and HST data, and the absolute chromospheric flux from the Halpha line core. From careful spectral analysis and the comparison of different methods we propose for kap^1 Cet the following atmospheric parameters: Teff = 5665+/-30 K (Halpha profile and energy distribution), log g = 4.49+/-0.05 dex (evolutionary and spectroscopic) and [Fe/H] = +0.10+/-0.05 dex (Fe II lines). The UV radiative properties of kap^1 Cet indicate that its flux is some 35% lower than the current Sun's between 210 and 300 nm, it matches the Sun's at 170 nm and increases to at least 2-7 times higher than the Sun's between 110 and 140 nm. The use of several indicators ascribes an age to kap^1 Cet in the interval ~0.4-0.8 Gyr and the analysis of the theoretical HR diagram suggests a mass ~1.04 Msun. This star is thus a very close analog of the Sun when life arose on Earth and Mars is thought to have lost its surface bodies of liquid water. Photochemical models indicate that the enhanced UV emission leads to a significant increase in photodissociation rates compared with those commonly assumed of the early Earth. Our results show that reliable calculations of the chemical composition of early planetary atmospheres need to account for the stronger solar photodissociating UV irradiation., Comment: 12 pages, accepted for publication in The Astrophysical Journal

The evolution of white dwarfs can be described as a simple cooling process. Recently, it has been possible to determine with an unprecedented precision their luminosity function, that is, the number of stars per unit volume and luminosity interval. Since the shape of the bright branch of this function is only sensitive to the average cooling rate, we use this property to check the possible existence of axions, a proposed but not yet detected weakly interacting particle. We show here that the inclusion of the axion emissivity in the evolutionary models of white dwarfs noticeably improves the agreement between the theoretical calculations and the observational white dwarf luminosity function, thus providing the first positive indication that axions could exist. Our results indicate that the best fit is obtained for m_a cos^2beta ~ 2-6 meV, where m_a is the mass of the axion and cos^2beta is a free parameter, and that values larger than 10 meV are clearly excluded., Comment: 8 pages, 7 figures, to appear in Journal of Physics Conference Proceedings for the 16th European White Dwarf Workshop

In this article we describe the search for white dwarfs (WDs) in the multi-band photometric data of the Capodimonte deep field survey. The WD candidates were selected through the V-R_C vs B-V color-color diagram. For two bright objects, the WD nature has been confirmed spectroscopically, and the atmospheric parameters (Teff and logg) have been determined. We have computed synthetic stellar population models for the observed field and the expected number of white dwarfs agrees with the observations. The possible contamination by turn-off and horizontal branch halo stars has been estimated. The quasar (QSO) contamination has been determined by comparing the number of WD candidates in different color bins with state-of-the-art models and previous observations. The WD space density is measured at different distances from the Sun. The total contamination (non-degenerate stars + QSOs) in our sample is estimated to be around 30%. This work should be considered a small experiment in view of more ambitious projects to be performed in the coming years in larger survey contexts., Comment: 9 pages, accepted for publication in Astronomy and Astrophysics (21/11/2008)

White dwarfs are the end-product of the lifes of intermediate- and low-mass stars and their evolution is described as a simple cooling process. Recently, it has been possible to determine with an unprecedented precision their luminosity function, that is, the number of stars per unit volume and luminosity interval. We show here that the shape of the bright branch of this function is only sensitive to the averaged cooling rate of white dwarfs and we propose to use this property to check the possible existence of axions, a proposed but not yet detected weakly interacting particle. Our results indicate that the inclusion of the emission of axions in the evolutionary models of white dwarfs noticeably improves the agreement between the theoretical calculations and the observational white dwarf luminosity function. The best fit is obtained for m_a cos^2 \beta ~ 5 meV, where m_a is the mass of the axion and cos^2 \beta is a free parameter. We also show that values larger than 10 meV are clearly excluded. The existing theoretical and observational uncertainties do not allow yet to confirm the existence of axions, but our results clearly show that if their mass is of the order of few meV, the white dwarf luminosity function is sensitive enough to detect their existence., Comment: 4 pages, 4 figures, replaced with version accepted by The Astrophysical Journal Letters

The initial-final mass relationship connects the mass of a white dwarf with the mass of its progenitor in the main-sequence. Although this function is of fundamental importance to several fields in modern astrophysics, it is not well constrained either from the theoretical or the observational points of view. In this work we revise the present semi-empirical initial-final mass relationship by re-evaluating the available data. The distribution obtained from grouping all our results presents a considerable dispersion, which is larger than the uncertainties. We have carried out a weighted least-squares linear fit of these data and a careful analysis to give some clues on the dependence of this relationship on some parameters such as metallicity or rotation. The semi-empirical initial-final mass relationship arising from our study covers the range of initial masses from 1.0 to 6.5 M_sun, including in this way the low-mass domain, poorly studied until recently. Finally, we have also performed a test of the initial-final mass relationship by studying its effect on the luminosity function and on the mass distribution of white dwarfs. This was done by using different initial-final mass relationships from the literature, including the expression derived in this work, and comparing the results obtained with the observational data from the Palomar Green Survey and the Sloan Digital Sky Survey (SDSS). We find that the semi-empirical initial-final mass relationship derived here gives results in good agreement with the observational data, especially in the case of the white dwarf mass distribution., Comment: 16 pages, 10 figures, accepted for publication in MNRAS

G39-27/289 is a common proper motion pair formed by a white dwarf (WD0433+270) and a main-sequence star (BD+26 730) that apparently has been classified as a member of the Hyades open cluster. Previous studies of the white dwarf component yielded a cooling time of ~4 Gyr. Although it has not been pointed out before explicitly, this result is 6 times larger than the age of the Hyades cluster, giving rise to an apparent conflict between the physics of white dwarfs and cluster main-sequence fitting. We investigate whether this system belongs to the Hyades cluster and, accordingly, give a plausible explanation to the nature of the white dwarf member. From kinematic and chemical composition considerations we believe that the system was a former member of the Hyades cluster and therefore has an evolutionary link with it. However, the evidence is not conclusive. With regards to the nature of the white dwarf component, we find that two core compositions (C/O and Fe) are compatible with the observed effective temperature and radius. These compositions yield very different cooling times of ~4 Gyr and ~1 Gyr, respectively. We distinguish two posssible scenarios. If the pair does not belong to the Hyades cluster but only to the Hyades stream, this would indicate that such stream contains rather old objects and definitely not coeval with the cluster. This has interesting consequences for Galactic dynamics. On the contrary, if the white dwarf has an Fe core, it will have a cooling time compatible with the Hyades age. This is a tantalizing result that would have implications for the thermonuclear explosion of white dwarfs and explosion theories of degenerate nuclei., Comment: 6 pages, 2 figures, accepted for publication in A&A

The initial-final mass relationship of white dwarfs, which is poorly constrained, is of paramount importance for different aspects in modern astrophysics. From an observational perspective, most of the studies up to now have been done using white dwarfs in open clusters. In order to improve the initial-final mass relationship we explore the possibility of deriving a semi-empirical relation studying white dwarfs in common proper motion pairs. We have acquired long-slit spectra of the white dwarf members of the selected common proper motion pairs, as well as high resolution spectra of their companions. From these observations, a full analysis of the two members of each common proper motion pair has lead to the initial and final masses of the white dwarfs. These observations have allowed us to provide updated information for the white dwarfs, since some of them were misclassified. This work is the first one in using common proper motion pairs to improve the initial-final mass relationship, and has also allowed to cover the poorly explored low-mass domain. As in the case of studies based on white dwarfs in open clusters, the distribution of the semi-empirical data presents a large scatter, which is higher than the expected uncertainties in the derived values. This suggests that the initial-final mass relationship may not be a single-valued function., Comment: 12 pages, 5 figures, accepted for publication in A&A