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Register a new userBrown Dwarfs: Toward a New Age Scale for Young Open Cluster
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Added by
David Barrado y Navascues
Publication date
1999
fields
Physics
and research's language is
English
Authors
D. Barrado y Navascues CfA
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We review our the status of the photometric searches of brown dwarfs and vely low mass stars in several young open clusters and present some results on the spectroscopic follow-up, including new cluster ages based on the position of the lithium depletion boundary.
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We have carried out multi-epoch, time-series differential I-band photometry of a large sample of objects in the south-east region of the young (~3 Myr), nearby (~350 pc) sigma Orionis open cluster. A field of ~1000 arcmin^2 was monitored during four nights over a period of two years. Using this dataset, we have studied the photometric variability of twenty-eight brown dwarf cluster candidates with masses ranging from the stellar-substellar boundary down to the planetary-mass domain. We have found that about 50% of the sample show photometric variability on timescales from less than one hour to several days and years. The amplitudes of the I-band light curves range from less than 0.01 up to ~0.4 magnitudes. A correlation between the near-infrared excess in the K_s band, strong Halpha emission and large-amplitude photometric variation is observed. We briefly discuss how these results may fit the different scenarios proposed to explain the variability of cool and ultracool dwarfs (i.e. magnetic spots, patchy obscuration by dust clouds, surrounding accretion discs and binarity). Additionally, we have determined tentative rotational periods in the range 3 to 40 h for three objects with masses around 60 M_Jup, and the rotational velocity of 14+/-4 km/s for one of them.
We present intermediate and low resolution optical spectroscopy (650-915 nm) of seven faint, very red objects (20 > I >= 17.8, I-Z >= 0.5) discovered in a CCD-based IZ survey covering an area of 1 sq. deg in the central region of the Pleiades open cluster. The observed spectra show that these objects are very cool dwarfs, having spectral types in the range M6-M9. Five out of the seven objects can be considered as Pleiades members on the basis of their radial velocities, Halpha emission and other gravity sensitive atomic features like the NaI doublet at 818.3 and 819.5 nm. According to current evolutionary models the masses of these new objects range from roughly 80 MJup for the hottest in the sample down to 45 MJup for Roque 4, the coolest and faintest confirmed member. These observations prove that the cloud fragmentation process extends well into the brown dwarf realm, suggesting a rise in the initial mass function below the substellar limit.
We present the results of a nonadiabatic, linear stability analysis of models of very low-mass stars (VLMSs) and brown dwarfs (BDs) during the deuterium burning phase in the center. We find unstable fundamental modes with periods varying between ~5 hr for a 0.1 Msun star and ~1 hr for a 0.02 Msun BD. The growth time of the instability decreases with decreasing mass and remains well below the deuterium burning time scale in the mass range considered (0.1--0.02 Msun). These results are robust against variations of the relevant input physics in the evolutionary models. We identify possible candidates for pulsational variability among known VLMSs and BDs in nearby star forming regions whose location in the HR diagram falls within or close to the boundary of the instability strip. Finally, we discuss the possibility that the variability observed in a few objects with periods of ~1 hr can be interpreted in terms of pulsation.
We have identified a large number of possible very low mass members of the cluster IC2391 based primarily on their location in an I versus (R-I)_C CM diagram. We have obtained new photometry and low resolution ($Delta lambda = 2.7$ AA) spectroscopy of 19 of these objects (14.9 $le$ I_C $le$ 17.5) in order to confirm cluster membership. We identify 15 of our targets as likely cluster members based on their spectral types, radial velocity, EW(NaI8200AA), and H(alpha) emission strengths. One of these stars has a definite lithium detection and two other (fainter) stars have possible lithium detections. We find the lithium depletion boundary in IC2391 is at I_C=16.2, which implies an age for IC2391 of 53$pm$5 Myr. While this is considerably older than the age most commonly attributed for this cluster (~35 Myr), the correction factor to the IC2391 age is comparable to those recently derived for the Pleiades and alpha Per clusters and can be explained by new models for high mass stars that incorporate a modest amount of convective core overshooting.
We present an improved, hybrid CPU-GPU atmospheric retrieval code, Helios-r2, which is applicable to medium-resolution emission spectra of brown dwarfs, in preparation for precision atmospheric spectroscopy in the era of the James Webb Space Telescope. The model is available as open-source code on the Exoclimes Simulation Platform. We subject Helios-r2 to a battery of tests of varying difficulty. The simplest test involves a mock retrieval on a forward model generated using the same radiative transfer technique, the same implementation of opacities, and the same chemistry model. The least trivial test involves a mock retrieval on synthetic spectra from the Sonora model grid, which uses a different radiative transfer technique, a different implementation of opacities, and a different chemistry model. A calibration factor, which is included to capture uncertainties in the brown dwarf radius, distance to the brown dwarf and flux calibration of the spectrum, may compensate, sometimes erroneously, for discrepancies in modeling choices and implementation. We analyze spectra of the benchmark brown dwarf GJ 570 D and the binary brown dwarf companions in the Epsilon Indi system. The retrieved surface gravities are consistent with previous studies and/or values inferred from dynamical masses (for Epsilon Indi Ba and Bb only). There remains no clear criterion on how to reject unphysical values of the retrieved brown dwarf radii. The inferred radii and corresponding masses should be taken with great caution. The retrieved carbon-to-oxygen ratios and metallicity depend on whether chemical equilibrium is assumed.
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