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Register a new userSpectroscopically confirmed brown dwarf members of Coma Berenices and the Hyades
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We have obtained low and medium resolution spectra of 9 brown dwarf candidate members of Coma Berenices and the Hyades using SpEX on the NASA InfaRed Telescope Facility and LIRIS on the William Herschel Telescope. We conclude that 7 of these objects are indeed late M or early L dwarfs, and that two are likely members of Coma Berenices, and four of the Hyades. Two objects, cbd40 and Hy3 are suggested to be a field L dwarfs, although there is also a possibility that Hy3 is an unresolved binary belonging to the cluster. These objects have masses between 71 and 53 M$_{rm Jup}$, close to the hydrogen burning boundary for these clusters, however only an optical detection of Lithium can confirm if they are truly substellar.
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We have identified stellar and substellar members in the nearby star cluster Coma Berenices, using photometry, proper motions, and distances of a combination of 2MASS, UKIDSS, URAT1, and {it Gaia}/DR2 data. Those with {it Gaia}/DR2 parallax measurements provide the most reliable sample to constrain the distance, averaging 86.7~pc with a dispersion 7.1~pc, and the age $sim800$~Myr, of the cluster. This age is older than the 400--600~Myr commonly adopted in the literature. Our analysis, complete within 5deg of the cluster radius, leads to identification of 192 candidates, among which, after field contamination is considered, about 148 are true members. The members have $Jsim3$~mag to $sim17.5$~mag, corresponding to stellar masses 2.3--0.06~$M_odot$. The mass function of the cluster peaks around 0.3~$M_odot$ and, in the sense of $dN/dm = m^{-alpha}$, where $N$ is the number of members and $m$ is stellar mass, has a slope $alphaapprox 0.49pm0.03$ in the mass range 0.3--2.3~$M_odot$. This is much shallower than that of the field population in the solar neighborhood. The slope $alpha=-1.69pm0.14$ from 0.3~$M_odot$ to 0.06~$M_odot$, the lowest mass in our sample. The cluster is mass segregated and has a shape elongated toward the Galactic plane. Our list contains nine substellar members, including three new discoveries of an M8, an L1 and an L4 brown dwarfs, extending from the previously known coolest members of late-M types to even cooler types.
We present spectroscopic follow-up observations of 68 red, faint candidates from our multi-epoch, multi-wavelength, previously published survey of NGC 2264. Using near-infrared spectra from VLT/KMOS, we measure spectral types and extinction for 32 young low-mass sources. We confirm 13 as brown dwarfs in NGC 2264, with spectral types between M6 and M8, corresponding to masses between 0.02 and 0.08$M_{odot}$. These are the first spectroscopically confirmed brown dwarfs in this benchmark cluster. 19 more objects are found to be young M-type stars of NGC 2264 with masses of 0.08 to 0.3$,M_{odot}$. 7 of the confirmed brown dwarfs as well as 15 of the M-stars have IR excess caused by a disc. Comparing with isochrones, the typical age of the confirmed brown dwarfs is $<$0.5 to 5Myr. More than half of the newly identified brown dwarfs and very low mass stars have ages $<$0.5Myr, significantly younger than the bulk of the known cluster population. Based on the success rate of our spectroscopic follow-up, we estimate that NGC 2264 hosts 200-600 brown dwarfs in total (in the given mass range). This would correspond to a star-to-brown dwarf ratio between 2.5:1 and 7.5:1. We determine the slope of the substellar mass function as $alpha = 0.43^{+0.41}_{-0.56}$, these values are consistent with those measured for other young clusters. This points to a uniform substellar mass function across all star forming environments.
We report the identification, from a photometric, astrometric and spectroscopic study, of a massive white dwarf member of the nearby, approximately solar metalicity, Coma Berenices open star cluster (Melotte 111). We find the optical to near-IR energy distribution of WD1216+260 to be entirely consistent with that of an isolated DA and determine the effective temperature and surface gravity of this object to be $T_{rm eff}$=$15739^{+197}_{-196}$K and log $g$=$8.46^{+0.03}_{-0.02}$. We set tight limits on the mass of a putative cool companion, M$simgreat$0.036M$_{odot}$ (spatially unresolved) and M$simgreat$0.034M$_{odot}$, (spatially resolved and a$simless$2500AU). Based on the predictions of CO core, thick-H layer evolutionary models we determine the mass and cooling time of WD1216+260 to be M$_{rm WD}$=$0.90 pm0.04$M$_{odot}$ and $tau$$_{rm cool}$=$363^{+46}_{-41}$Myrs respectively. For an adopted cluster age of $tau$=500$pm$100Myrs we infer the mass of its progenitor star to be M$_{rm init}$=$4.77^{+5.37}_{-0.97}$M$_{odot}$. We briefly discuss this result in the context of the form of the stellar initial mass-final mass relation.
We present the results of a survey of the Coma Berenices open star cluster (Melotte 111), undertaken using proper motions from the USNO-B1.0 and photometry from the 2MASS Point Source catalogues. We have identified 60 new candidate members with masses in the range 1.007<M<$0.269M_solar. For each we have estimated a membership probability by extracting control clusters from the proper motion vector diagram. All 60 are found to have greater than 60 per cent probability of being clusters more than doubling the number of known cluster members. The new luminosity function for the cluster peaks at bright magnitudes, but is rising at K~12, indicating that it is likely lower mass members may exist. The mass function also supports this hypothesis.
From the luminosity, effective temperature, and age of the Hyades brown dwarf 2MASSJ04183483+2131275 (2M0418), sub-stellar evolutionary models predict a mass in the range 39-55 Jupiter masses (M_Jup) which is insufficient to produce any substantial lithium burning except for the very upper range >53 M_Jup. Our goal is to measure the abundance of lithium in this object, test the consistency between models and observations and refine constraints on the mass and age of the object. We used the 10.4-m Gran Telescopio Canarias (GTC) with its low-dispersion optical spectrograph to obtain ten spectra of 2277s each covering the range 6300-10300 Angstroms with a resolving power of R~500. In the individual spectra, which span several months, we detect persistent unresolved H_alpha in emission with pseudo equivalent widths (pEW) in the range 45-150 Angstroms and absorption lines of various alkalis with the typical strengths found in objects of L5 spectral type. The lithium resonance line at 6707.8 Angstroms is detected with pEW of 18+/-4 Angstroms in 2M0418 (L5). We determine a lithium abundance of log N(Li) = 3.0+/-0.4 dex consistent with a minimum preservation of 90% of this element which confirms 2M0418 as a brown dwarf with a maximum mass of 52 M_Jup. We infer a maximum age for the Hyades of 775 Myr from a comparison with the BHAC15 models. Combining recent results from the literature with our study, we constrain the mass of 2M0418 to 45-52 M_Jup and the age of the cluster to 580-775 Myr (1 sigma) based on the lithium depletion boundary method.
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