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Register a new userCharacterization of Stellar and Substellar Members in the Coma Berenices Star Cluster
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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.
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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<$0.269M_solar. For each we have estimated a membership probability by extracting control clusters from the proper motion vector diagram. All 60 are found to have greater than 60 per cent probability of being clusters more than doubling the number of known cluster members. The new luminosity function for the cluster peaks at bright magnitudes, but is rising at K~12, indicating that it is likely lower mass members may exist. The mass function also supports this hypothesis.
We have obtained low and medium resolution spectra of 9 brown dwarf candidate members of Coma Berenices and the Hyades using SpEX on the NASA InfaRed Telescope Facility and LIRIS on the William Herschel Telescope. We conclude that 7 of these objects are indeed late M or early L dwarfs, and that two are likely members of Coma Berenices, and four of the Hyades. Two objects, cbd40 and Hy3 are suggested to be a field L dwarfs, although there is also a possibility that Hy3 is an unresolved binary belonging to the cluster. These objects have masses between 71 and 53 M$_{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.
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 a study of metallicities in a sample of main sequence stars with spectral types M, K, G and F ($T_{rm eff}$ $sim$ 3200 -- 6500K and log $g$ $sim$ 4.3 -- 5.0 dex) belonging to the solar neighborhood young open cluster Coma Berenices. Metallicities were determined using the high-resolution (R=$lambda$/$Delta$ $lambda$ $sim$ 22,500) NIR spectra ($lambda$1.51 -- $lambda$1.69 $mu$m) of the SDSS-IV APOGEE survey. Membership to the cluster was confirmed using previous studies in the literature along with APOGEE radial velocities and Gaia DR2. An LTE analysis using plane-parallel MARCS model atmospheres and the APOGEE DR16 line list was adopted to compute synthetic spectra and derive atmospheric parameters ($T_{rm eff}$ and log $g$) for the M dwarfs and metallicities for the sample. The derived metallicities are near solar and are homogeneous at the level of the expected uncertainties, in particular when considering stars from a given stellar class. The mean metallicity computed for the sample of G, K, and M dwarfs is $langle$[Fe/H]$rangle$ = +0.04 $pm$ 0.02 dex; however, the metallicities of the F-type stars are slightly lower, by about 0.04 dex, when compared to cooler and less massive members. Models of atomic diffusion can explain this modest abundance dip for the F dwarfs, indicating that atomic diffusion operates in Coma Berenices stars. The [Fe/H] dip occurs in nearly the same effective temperature range as that found in previous analyses of the lithium and beryllium abundances in Coma Berenices.
We present the results of a photometric survey of rotation rates in the Coma Berenices (Melotte 111) open cluster, using data obtained as part of the SuperWASP exoplanetary transit-search programme. The goal of the Coma survey was to measure precise rotation periods for main-sequence F, G and K dwarfs in this intermediate-age (~600 Myr) cluster, and to determine the extent to which magnetic braking has caused the stellar spin periods to converge. We find a tight, almost linear relationship between rotation period and J-K colour with a root-mean square scatter of only 2 percent. The relation is similar to that seen among F, G and K stars in the Hyades. Such strong convergence can only be explained if angular momentum is not at present being transferred from a reservoir in the deep stellar interiors to the surface layers. We conclude that the coupling timescale for angular momentum transport from a rapidly-spinning radiative core to the outer convective zone must be substantially shorter than the cluster age, and that from the age of Coma onward, stars rotate effectively as solid bodies. The existence of a tight relationship between stellar mass and rotation period at a given age supports the use of stellar rotation period as an age indicator in F, G and K stars of Hyades age and older. We demonstrate that individual stellar ages can be determined within the Coma population with an internal precision of order 9 percent (RMS), using a standard magnetic braking law in which rotation period increases with the square root of stellar age. We find that a slight modification to the magnetic-braking power law, P proportional to t^0.56, yields rotational and asteroseismological ages in good agreement for the Sun and other stars of solar age for which p-mode studies and photometric rotation periods have been published.
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