ABSTRACT
Over the past decades open clusters have been the subject of many studies. Such studies are crucial considering that the universality of the initial mass function is still a subject of ...current investigations. Praesepe is an interesting open cluster for the study of the stellar and substellar mass function (MF), considering its intermediate age and its nearby distance. Here we present the results of a wide‐field, near‐infrared study of Praesepe using the Data Release 9 of the United Kingdom Infrared Telescope Infrared Deep Sky Survey Galactic Clusters Survey. We obtained cluster candidates of Praesepe based on a 3σ astrometric and five‐band photometric selection. We derived a binary frequency for Praesepe of 25.6 ± 3.0 per cent in the 0.2–0.45 M⊙ mass range, 19.6 ± 3.0 per cent for 0.1–0.2 M⊙ and 23.2 ± 5.6 per cent for 0.07–0.1 M⊙. We also studied the variability of the cluster candidates of Praesepe, and we conclude that seven objects could be variable. We inferred the luminosity function of Praesepe in the Z and J bands and derived its MF. We observe that our determination of the MF of Praesepe differs from previous studies: while previous MFs present an increase from 0.6 to 0.1 M⊙, our MF shows a decrease. We looked at the MF of Praesepe in two different regions of the cluster, i.e. within and beyond 1°.25, and we observed that both regions present an MF which decrease to lower masses. We compared our results with the Hyades, the Pleiades and α Per MF in the mass range 0.072–0.6 M⊙ and showed that the Praesepe MF is more similar to α Per although they are, respectively, of ages ∼85 and ∼600 Myr. Even though of similar age, the Praesepe remains different than the Hyades, with a decrease in the MF of only ∼0.2 dex from 0.6 down to 0.1 M⊙, compared to ∼1 dex for the Hyades.
We have obtained a low-resolution optical spectrum for one of the faintest cluster member candidates in Praesepe with the Optical System for Imaging and low Resolution Integrated Spectroscopy mounted ...on the 10.4 m Gran Telescopio de Canarias. We confirm spectroscopically the first L dwarf member in Praesepe, UGCS J084510.66+214817.1. We derived an optical spectral type of L0.3±0.4 and estimated its effective temperature to T
eff = 2279 ± 371 K and a mass of 71.1 ± 23.0 M
Jup, according to state-of-the-art models, placing it at the hydrogen-burning boundary. We measured the equivalent width of the gravity-sensitive sodium doublet at 8182/8194 Å, which adds credit to the membership of this new L dwarf to Praesepe. We also derived a probability of ∼20.5 per cent that our candidate would be a field L0 dwarf. We conclude that this object is likely to be a true member of Praesepe, with evidence of being a binary system.
ABSTRACT
In recent years, some extremely red brown dwarfs have been discovered. They were believed to have a low surface gravity, but many of their spectral characteristics are similar to those of ...high-surface-gravity brown dwarfs, showing that the spectral characteristics of young brown dwarfs are poorly understood. We aim to test surface-gravity indicators in late-M and early-L brown dwarf spectra using data obtained with the X-shooter spectrograph at the Very Large Telescope. We select a benchmark sample of brown dwarf members of Chamaeleon I (∼2 Myr), Upper Scorpius (5−10 Myr), the Pleiades (132 ± 27 Myr) and Praesepe (590−790 Myr) with well-constrained ages and similar metallicities. We provide a consistent spectral classification of the sample in the optical and in the near-infrared. We measure the equivalent widths of their alkali lines, finding that they have a moderate correlation with age, especially for objects with spectral types M8 and later. We use spectral indices defined in the literature to estimate surface gravity, finding that their gravity assignment is accurate for 75 per cent of our sample. We investigate the correlation between red colour and age, finding that after ∼10 Myr, the colour does not change significantly for our sample with spectral types M6.0–L3.0. In this case, the red colours might be associated with circumstellar discs, ring structures, extinction, or viewing angle. Finally, we calculate the bolometric luminosity, and J and K bolometric corrections for our sample. We find that six objects are overluminous compared with other members of the same association. These objects are flagged as binary candidates by the Gaia survey.
Spectroscopy of Hyades L dwarf candidates Lodieu, N; Boudreault, S; Béjar, V. J. S
Monthly Notices of the Royal Astronomical Society,
12/2014, Volume:
445, Issue:
4
Journal Article
Peer reviewed
Open access
We present the results of photometric, astrometric, and spectroscopic follow-up of L dwarf candidates identified in the Hyades cluster by Hogan et al. We obtained low-resolution optical spectroscopy ...with the Optical System for Imaging and low-intermediate Resolution Integrated Spectroscopy spectrograph on the Gran Telescopio de Canarias for all 12 L dwarf candidates as well as new J-band imaging for a subsample of 8 to confirm their proper motion. We also present mid-infrared photometry from the Wide Field Infrared Survey Explorer for the Hyades L and T dwarf candidates and estimate their spectroscopic distances, effective temperatures, and masses. We confirm the cool nature of several L dwarf candidates and confirm astrometrically their membership, bridging the gap between the coolest M dwarfs and the two T dwarfs previously reported in the Hyades cluster. These members represent valuable spectral templates at an age of 625 Myr and slightly supersolar metallicity (Fe/H = +0.13). We update the Hyades mass function across the hydrogen-burning limit and in the substellar regime. We confirm a small number numbers of very low mass members below ∼0.1 M⊙ belonging to the Hyades cluster.
ABSTRACT
We present the results of a deep (J = 19.1 mag) infrared (ZY JHK) survey over the full α Per open cluster extracted from the Data Release 9 of the United Kingdom Infrared Telescope Infrared ...Deep Sky Survey Galactic Clusters Survey (UKIDSS). We have selected ∼700 cluster member candidates in ∼56 square degrees in α Per by combining photometry in five near‐infrared passbands and proper motions derived from the multiple epochs provided by the UKIDSS Galactic Clusters Survey (GCS) Data Release 9 (DR9). We also provide revised membership for all previously published α Per low‐mass stars and brown dwarfs recovered in GCS based on the new photometry and astrometry provided by DR9. We find no evidence of K‐band variability in members of α Per with dispersion less than 0.06–0.09 mag. We employed two independent but complementary methods to derive the cluster luminosity and mass functions: a probabilistic analysis and a more standard approach consisting of stricter astrometric and photometric cuts. We find that the resulting luminosity and mass functions obtained from both methods are consistent. We find that the shape of the α Per mass function is similar to that of the Pleiades although the characteristic mass may be higher after including higher mass data from earlier studies (the dispersion is comparable). We conclude that the mass functions of α Per, the Pleiades and Praesepe are best reproduced by a log‐normal representation similar to the system field mass function although with some variation in the characteristic mass and dispersion values.
ABSTRACT
We present Keck I/OSIRIS and Keck II/NIRC2 adaptive optics imaging of two member candidates of the Praesepe stellar cluster (d = 186.18 ± 0.11 pc; 590–790 Myr), UGCS J08451066+2148171 ...(L1.5 ± 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 ± 0.04 and a projected separation of 60.3 ± 1.3 mas (11.2 ± 0.7 au; 1σ). We also resolved UGCS J08301935+2003293 into a binary system with a flux ratio of 0.46 ± 0.03 and a separation of 62.5 ± 0.9 mas. Assuming zero eccentricity, we estimate minimum orbital periods of ∼100 yr for both systems. According to theoretical evolutionary models, we derive masses in the range of 0.074–0.078 and 0.072–0.076 M⊙ for the primary and secondary of UGCS J08451066+2148171 for an age of 700 ± 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⊙), while the companion candidate might be a brown dwarf (0.051–0.065 M⊙). 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.
Aims. We aim at identifying the least massive population of the solar metallicity, young (120 Myr), nearby (133.5 pc) Pleiades star cluster with the ultimate goal of understanding the physical ...properties of intermediate-age, free-floating, low-mass brown dwarfs and giant planetary-mass objects, and deriving the cluster substellar mass function across the deuterium-burning mass limit at ≈0.012 M⊙. Methods. We performed a deep photometric and astrometric J- and H-band survey covering an area of ~0.8 deg2 in the Pleiades cluster. The images with completeness and limiting magnitudes of J,H ≈ 20.2 and ≈21.5 mag were acquired ~9 yr apart, allowing us to derive proper motions with a typical precision of ±6 mas yr-1. For the cluster distance and age, the survey is sensitive to Pleiades members with masses in the interval ≈0.2−0.008 M⊙. J- and H-band data were complemented with Z, K, and mid-infrared magnitudes up to 4.6 μm coming from the UKIRT Infrared Deep Sky Survey (UKIDSS), the WISE catalog, and follow-up observations of our own. Pleiades member candidates were selected to have proper motions compatible with that of the cluster, and colors following the known Pleiades sequence in the interval J = 15.5−18.8 mag, and ZUKIDSS − J ≥ 2.3 mag or Z nondetections for J> 18.8 mag. Results. We found a neat sequence of astrometric and photometric Pleiades substellar member candidates with two or more proper motion measurements and with magnitudes and masses in the intervals J = 15.5−21.2 mag and ≈0.072−0.008 M⊙. The faintest objects show very red near- and mid-infrared colors exceeding those of field high-gravity dwarfs by ≥0.5 mag. This agrees with the reported properties of field young L-type dwarfs and giant planets orbiting stars of ages of ~100 Myr. The Pleiades photometric sequence does not show any color turn-over because of the presence of photospheric methane absorption down to J = 20.3 mag, which is about 1 mag fainter than predicted by the combination of evolutionary models and colors computed from model atmospheres. The astrometric data suggest that Pleiades brown dwarfs have a proper motion dispersion of 6.4−7.5 mas yr-1, and are dynamically relaxed at the age of the cluster. The Pleiades mass function extends down to the deuterium burning-mass threshold, with a slope fairly similar to that of other young star clusters and stellar associations. The new discoveries may become benchmark objects for interpreting the observations of the emerging young ultracool population and giant planets around stars in the solar neighborhood.
We report on the near-infrared (JHK-bands) low-resolution spectroscopy and red optical (Z-band) photometry of seven proper-motion, very low-mass substellar member candidates of the Pleiades cluster ...with magnitudes in the interval J = 17.5−20.8 and K = 16.1−18.5 mag. Spectra were acquired for six objects with the LIRIS and NIRSPEC instruments mounted on the 4.2-m William Herschel and the 10-m Keck II telescopes, respectively. Z-band images of two of the faintest candidates were collected with the ACAM instrument on the WHT. The new data confirm the low temperatures of all seven Pleiades proper motion candidates. From the imaging observations, we find extremely red Z − J and Z − K colors that suggest that the faintest target, Calar Pleiades 25, has a Galactic rather than extragalactic nature. We tentatively classify the spectroscopic targets from early-L to ~T0 and suggest that the L/T transition, which accounts for the onset of methane absorption at 2.1 μm, may take place at J ≈ 20.3 and K ≈ 17.8 mag in the Pleiades (absolute values of MJ ≈ 14.7 and MK ≈ 12.2 mag). We find evidence of likely low-gravity atmospheres based on the presence of triangular-shape H-band fluxes and the high flux ratio K/H (compatible with red H − K colors) of Calar Pleiades 20, 21, and 22, which is a feature also seen in field low-gravity dwarfs. Weak K i absorption lines at around 1.25 μm are probably seen in two targets. These observations add support to the cluster membership of all seven objects in the Pleiades. The trend delineated by the spectroscopic sequence of Pleiades late-M and L dwarfs resembles that of the field. With masses estimated at 0.012−0.015 M⊙ (solar metallicity and 120 Myr), Calar Pleiades 20 (L6±1), 21 (L7±1), and 22 (L/T) may become the coolest and least massive Pleiades members that are corroborated with photometry, astrometry, and spectroscopy. Calar Pleiades 25 (<0.012 M⊙) is a firm free-floating planetary-mass candidate in the Pleiades.
Gaia Data Release 2 Hambly, N. C.; Cropper, M.; Boudreault, S. ...
Astronomy & astrophysics,
08/2018, Volume:
616
Journal Article
Peer reviewed
Open access
Context. The European Space Agency’s Gaia satellite was launched into orbit around L2 in December 2013. This ambitious mission has strict requirements on residual systematic errors resulting from ...instrumental corrections in order to meet a design goal of sub-10 microarcsecond astrometry. During the design and build phase of the science instruments, various critical calibrations were studied in detail to ensure that this goal could be met in orbit. In particular, it was determined that the video-chain offsets on the analogue side of the analogue-to-digital conversion electronics exhibited instabilities that could not be mitigated fully by modifications to the flight hardware. Aims. We provide a detailed description of the behaviour of the electronic offset levels on short (<1 ms) timescales, identifying various systematic effects that are known collectively as “offset non-uniformities”. The effects manifest themselves as transient perturbations on the gross zero-point electronic offset level that is routinely monitored as part of the overall calibration process. Methods. Using in-orbit special calibration sequences along with simple parametric models, we show how the effects can be calibrated, and how these calibrations are applied to the science data. While the calibration part of the process is relatively straightforward, the application of the calibrations during science data processing requires a detailed on-ground reconstruction of the readout timing of each charge-coupled device (CCD) sample on each device in order to predict correctly the highly time-dependent nature of the corrections. Results. We demonstrate the effectiveness of our offset non-uniformity models in mitigating the effects in Gaia data. Conclusions. We demonstrate for all CCDs and operating instrument/modes on board Gaia that the video-chain noise-limited performance is recovered in the vast majority of science samples.
Gaia Data Release 2 Katz, D.; Sartoretti, P.; Cropper, M. ...
Astronomy and astrophysics (Berlin),
02/2019, Volume:
622
Journal Article
Peer reviewed
Open access
Context. For Gaia DR2, 280 million spectra collected by the Radial Velocity Spectrometer instrument on board Gaia were processed, and median radial velocities were derived for 9.8 million sources ...brighter than GRVS = 12 mag. Aims. This paper describes the validation and properties of the median radial velocities published in Gaia DR2. Methods. Quality tests and filters were applied to select those of the 9.8 million radial velocities that have the quality to be published in Gaia DR2. The accuracy of the selected sample was assessed with respect to ground-based catalogues. Its precision was estimated using both ground-based catalogues and the distribution of the Gaia radial velocity uncertainties. Results. Gaia DR2 contains median radial velocities for 7 224 631 stars, with Teff in the range 3550, 6900 K, which successfully passed the quality tests. The published median radial velocities provide a full-sky coverage and are complete with respect to the astrometric data to within 77.2% (for G ≤ 12.5 mag). The median radial velocity residuals with respect to the ground-based surveys vary from one catalogue to another, but do not exceed a few 100 m s−1. In addition, the Gaia radial velocities show a positive trend as a function of magnitude, which starts around GRVS ~ 9 mag and reaches about + 500 m s−1 at GRVS = 11.75 mag. The origin of the trend is under investigation, with the aim to correct for it in Gaia DR3. The overall precision, estimated from the median of the Gaia radial velocity uncertainties, is 1.05 km s−1. The radial velocity precision is a function of many parameters, in particular, the magnitude and effective temperature. For bright stars, GRVS ∈ 4, 8 mag, the precision, estimated using the full dataset, is in the range 220–350 m s−1, which is about three to five times more precise than the pre-launch specification of 1 km s−1. At the faint end, GRVS = 11.75 mag, the precisions for Teff = 5000 and 6500 K are 1.4 and 3.7 km s−1, respectively.
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