The GSC-II-based survey of ancient cool white dwarfs

Carollo, D.; Bucciarelli, B.; Hodgkin, S. T.; Lattanzi, M. G.; McLean, B.; Morbidelli, R.; Smart, R. L.; Spagna, A.; Terranegra, L.

doi:10.1051/0004-6361:20054113

Cited by 15 publications

(19 citation statements)

References 26 publications

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“…1) is often used to separate the bottom of the main sequence (MS), the cool subdwarf (sd) sequence and the CWD sequence, when trigonometric distances and absolute magnitudes are not yet available (see e.g. Kilic et al 2006;Carollo et al 2006;Vidrih et al 2007). The reduced proper motion (Hertzsprung 1905;Luyten 1922) is defined as H = m + 5 + 5 log μ, where m is the apparent magnitude and μ is the proper motion expressed in arcsec/yr.…”

Section: Reduced Proper Motion Diagram and I-magnitude Cutmentioning

confidence: 99%

“…CWDs have long been considered as important objects for a number of reasons, such as determining the age of the Galactic disk (Leggett et al 1998) and as possible components of the Galactic dark matter halo Oppenheimer et al 2001;Méndez 2002;Flynn et al 2003;Salim et al 2004;Reid 2005). Among the coolest known CWDs (T eff < 4500 K) in the Solar neighbourhood there are many objects already catalogued in the LHS (Luyten 1979a) or discovered as HPM objects from photographic plates (Hambly et al 1997;Harris et al 1999;Ibata et al 2000;Scholz et al 2000;Monet et al 2000;Scholz et al 2002;Carollo et al 2006;Rowell et al 2008). Other CWDs have been found in the SDSS (Harris et al 2001;Gates et al 2004;Kilic et al 2006;Vidrih et al 2007;Harris et al 2008).…”

Section: Introductionmentioning

confidence: 99%

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Extremely faint high proper motion objects from SDSS stripe 82

Scholz

Storm

Knapp³

et al. 2009

A&A

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Aims. By pushing the magnitude limit of high proper motion surveys beyond the limit of photographic Schmidt plates, we aim to discover nearby and very fast low-luminosity objects of different classes: cool white dwarfs (CWDs), cool subdwarfs (sd), and very low-mass stars and brown dwarfs at the very faint end of the main sequence (MS). Methods. The deep multi-epoch Sloan Digital Sky Survey data in a 275 square degree area along the celestial equator (SDSS stripe 82) allow us to search for extremely faint (i > 21) objects with proper motions greater than 0.14 arcsec/yr. A reduced proper motion diagram H z /(i − z) clearly reveals three sequences (MS, sd, CWD) where our faintest candidates are representative of the still poorly known bottom of each sequence. We classify 38 newly detected objects with low-resolution optical spectroscopy using FORS1 @ ESO VLT. Together with our targets we observe six known L dwarfs in stripe 82, four (ultra)cool sd and one CWD as comparison objects. Distances and tangential velocities are estimated using known spectral type/absolute magnitude relations. Results. All 22 previously known L dwarfs (and a few of the T dwarfs) in stripe 82 have been detected in our high proper motion survey. However, 11 of the known L dwarfs have smaller proper motions (0.01 < μ < 0.14 arcsec/yr). Although stripe 82 was already one of the best investigated sky regions with respect to L and T dwarfs, we are able to classify 13 new L dwarfs. Two previously known L dwarfs have been reclassified by us. We have also found eight new M 7.5-M 9.5 dwarfs. The four new CWDs discovered by us are about 1-2 mag fainter than those previously detected in SDSS data. All new L-type, late-M and CWD objects show thick disk and halo kinematics. Since our high-velocity late-M and L dwarfs do not show indications of low metallicity in their spectra, we conclude that there may be a population of ultracool halo objects with normal metallicities. There are 13 objects, mostly with uncertain proper motions, which we initially classified as mid-M dwarfs. Among them we have found 9 with an alternative subdwarf classification (sdM7 or earlier types), whereas we have not found any new spectra resembling the known ultracool (>sdM7) subdwarfs. Some M subdwarf candidates have been classified based on spectral indices with large uncertainties. Conclusions. We failed to detect new nearby (d < 50 pc) L dwarfs, probably because the SDSS stripe 82 area was already wellinvestigated before. With our survey we have demonstrated a higher efficiency in finding Galactic halo CWDs than previous searches. The space density of halo CWDs is according to our results about 1.5-3.0 × 10 −5 pc −3 .

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Section: Reduced Proper Motion Diagram and I-magnitude Cutmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Extremely faint high proper motion objects from SDSS stripe 82

Scholz

Storm

Knapp³

et al. 2009

A&A

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“…For example, only three out of the 112 white dwarfs reported by Kilic et al (2006) are detected in Y J H and lie within the LAS DR2 area, and only one of those met our color criteria. Other sources, which are not in our sky area, include those of Gates et al (2004);Hall et al (2008); Rowell et al (2008); and most of the Carollo et al (2006) and Harris et al (2008) sources. Those with inappropriate colors include the Harris et al (2001) source that is not detected at J or H; two Carollo et al sources; and two sources from Harris et al (2008), one of which is not detected at H and the other of which has J − H > 0.…”

Section: Sample Selectionmentioning

confidence: 91%

Cool White Dwarfs Identified in the Second Data Release of the Ukirt Infrared Deep Sky Survey

Lodieu

Leggett

Bergeron

et al. 2009

ApJ

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We have paired the second data release of the Large Area Survey of the UKIRT Infrared Deep Sky Survey with the fifth data release of the Sloan Digital Sky Survey to identify 10 cool white dwarf candidates, from their photometry and astrometry. Of these 10, one was previously known to be a very cool white dwarf. We have obtained optical spectroscopy for seven of the candidates using the GMOS-N spectrograph on Gemini North, and have confirmed all seven as white dwarfs. Our photometry and astrometry indicate that the remaining two objects are also white dwarfs. The model analysis of the photometry and available spectroscopy shows that the seven confirmed new white dwarfs, and the two new likely white dwarfs, have effective temperatures in the range of T eff = 5400-6600 K. Our analysis of the previously known white dwarf confirms that it is cool, with T eff = 3800 K. The cooling age for this dwarf is 8.7 Gyr, while that for the nine ∼ 6000 K white dwarfs is 1.8-3.6 Gyr. We are unable to determine the masses of the white dwarfs from the existing data, and therefore we cannot constrain the total ages of the white dwarfs. The large cooling age for the coolest white dwarf in the sample, combined with its low estimated tangential velocity, suggests that it is an old member of the thin disk, or a member of the thick disk of the Galaxy, with an age of 10-11 Gyr. The warmer white dwarfs appear to have velocities typical of the thick disk or even halo; these may be very old remnants of low-mass stars, or they may be relatively young thin-disk objects with unusually high space motion.

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“…Applying the Bayesian approach described in Section 2.2 and using the informative log g prior given in Section 4 giveT eff = 3400 ± 30 K for SDSS J224206+004822 (where H is a better fit but neither model fits the data well), andT eff = 4760 ± 20 K for SDSS J233055+002852 (where He is clearly a better fit). SDSS J232115.67+010223.8 was reported by Carollo et al (2006) as being of type DC; the Bayesian results imply that this star most probably has a He atmosphere, withT eff = 4870 ± 20 K. Only one of the 24 ultracool WD candidates identified by V07 was found to definitely have a He-dominated atmosphere. Given the strong degeneracies in the photometric WD fitting, it was possible that this conclusion came about only due to the sparse grid of H models 'straddling' the good fit locus while one of the He models fortuitously landed on it.…”

Section: Notes On Individual Objectsmentioning

confidence: 92%

Photometric constraints on white dwarfs and the identification of extreme objects

Mortlock

Peiris

Ivezić

2009

Monthly Notices of the Royal Astronomical Society

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It is possible to reliably identify white dwarfs (WDs) without recourse to spectra, instead using photometric and astrometric measurements to distinguish them from main-sequence stars and quasars. WDs' colours can also be used to infer their intrinsic properties (effective temperature, surface gravity, etc.), but the results obtained must be interpreted with care. The difficulties stem from the existence of a solid angle degeneracy, as revealed by a full exploration of the likelihood, although this can be masked if a simple best-fitting approach is used. Conversely, this degeneracy can be broken if a Bayesian approach is adopted, as it is then possible to utilize the prior information on the surface gravities of WDs implied by spectroscopic fitting. The benefits of such an approach are particularly strong when applied to outliers, such as the candidate halo and ultracool WDs identified by Vidrih et al. A reanalysis of these samples confirms their results for the latter sample, but suggests that most of the halo candidates are thick-disc WDs in the tails of the photometric noise distribution.

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The GSC-II-based survey of ancient cool white dwarfs

Cited by 15 publications

References 26 publications

Extremely faint high proper motion objects from SDSS stripe 82

Extremely faint high proper motion objects from SDSS stripe 82

Cool White Dwarfs Identified in the Second Data Release of the Ukirt Infrared Deep Sky Survey

Photometric constraints on white dwarfs and the identification of extreme objects

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