An optical and near-infrared search for brown dwarfs in the Pleiades cluster

We have carried out an extensive search for X-ray emission from young, very low-mass objects near and beyond the substellar limit, making use of archived ROSAT PSPC and HRI observations pointed at Brown Dwarfs and Brown Dwarf candidates in the young sigma Orionis and Taurus-Auriga associations. In sigma Ori we identify three Brown Dwarf candidates with X-ray sources; in Taurus-Auriga we add one further X-ray detection of a Brown Dwarf to the list published earlier. We combine this data with all previously X-ray detected Brown Dwarfs and Brown Dwarf candidates in young stellar associations and star forming regions to perform a study of stellar activity parameters on the as yet largest sample of young, very low mass objects. A similar relation between X-ray and bolometric luminosity, and H-alpha emission, respectively, as is known for T Tauri stars seems to hold for young objects down to the substellar limit, too. No signs for a change in X-ray activity are found on the transition to substellar masses.

We present the results of a high-resolution imaging survey for brown dwarf binaries in two open clusters. The observations were carried out with the Wide Field Planetary Camera 2 on board the Hubble Space Telescope. Our sample consists of eight brown dwarf candidates in α Persei and 25 brown dwarf candidates in the Pleiades. We have resolved four binaries in the Pleiades with separations in the range 0094-0058, corresponding to projected separations between 11.7 and 7.2 AU. No binaries were found among the α Per targets. Three of the binaries have proper motions consistent with cluster membership in the Pleiades cluster, and for one of them we report the detection of Hα in emission and Li I absorption obtained from Keck II/ESI spectroscopy. One of the binaries does not have a proper motion consistent with Pleiades membership. We estimate that brown dwarf binaries wider than 12 AU are less frequent than 9% in the α Per and Pleiades clusters. This is consistent with an extension to substellar masses of a trend observed among stellar binaries: the maximum semimajor axis of binary systems decreases with decreasing primary mass. We find a binary frequency of two binaries over 13 brown dwarfs with confirmed proper-motion membership in the Pleiades, corresponding to a binary fraction of 15%. These binaries are limited to the separation range 7-12 AU, and their mass ratios are larger than 0.7. The observed properties of Pleiades brown dwarf binaries appear to be similar to their older counterparts in the solar neighborhood. The relatively high binary frequency (≥10%), the bias to separations smaller than about 15 AU, and the trend to high mass ratios (q ≥ 0.7) are fundamental properties of brown dwarfs. Current theories of brown dwarf formation do not appear to provide a good description of all these properties.

We present near-infrared photometry and optical spectroscopy of very low mass stars and brown dwarf candidates in the Pleiades open cluster. The membership status of these objects is assessed using color-magnitude diagrams, lithium and spectral types. Eight objects out of 45 appear to be nonmembers. A search for companions among 34 very low mass Pleiades members (M ≤ 0.09 M☉) in high spatial resolution images obtained with the Hubble Space Telescope (HST) and the adaptive optics system of the Canada-France-Hawaii telescope produced no resolved binaries with separations larger than 02 (a ~ 27 AU; P ~ 444 yr). Nevertheless, we find evidence for a binary sequence in the color-magnitude diagrams, in agreement with the results of Steele & Jameson for higher mass stars. We apply the lithium test to two objects: CFHT-Pl-16, which lies in the cluster binary sequence but is unresolved in images obtained with the Hubble Space Telescope; and CFHT-Pl-18, which is binary with 033 separation. The first object passes the test, but the second object does not. We conclude that CFHT-Pl-16 is an Pleiades brown dwarf binary with separation less than 11 AU and that CFHT-Pl-18 is a foreground system. We compare the multiplicity statistics of the Pleiades very low mass stars and brown dwarfs with that of G- and K-type main-sequence stars in the solar neighborhood. We find that there is some evidence for a deficiency of wide binary systems (separation >27 AU) among the Pleiades very low mass members. We briefly discuss how this result can fit with current scenarios of brown dwarf formation. We correct the Pleiades substellar mass function for the contamination of cluster nonmembers found in this work. We find a contamination level of 33% among the brown dwarf candidates identified by Bouvier et al. Assuming a power-law IMF across the substellar boundary, we find a slope dN/dM ~ M-0.53, implying that the number of objects per mass bin is still rising but the contribution to the total mass of the cluster is declining in the brown dwarf regime.

view Abstract Citations (54) References (63) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Radial Velocities of Very Low mass Stars and Candidate Brown Dwarf Members of the Hyades and Pleiades Stauffer, John R. ; Liebert, James ; Giampapa, Mark ; Macintosh, Bruce ; Reid, Neill ; Hamilton, Donald Abstract We have determined H alpha equivalent widths and radial velocities with 1 sigma accuracies of approximately 5 km s-1 for approximately 20 candidate very low mass members of the Hyades and Pleiades clusters. The radial velocities for the Hyades sample suggest that nearly all of these stars are indeed highly probable members of the Hyades. The faintest stars in the Hyades sample have masses of order 0.1 solar mass. We also obtained radial velocities for four candidate very low mass members of the Pleiades and two objects that are candidate BD Pleiads. All of these stars have apparent V magnitudes fainter than the Hyades stars we observed, and the resultant radial velocity accuracy is worse. We believe that the three brighter stars are indeed likely very low mass stellar members of the Pleiades, whereas the status of the two brown dwarf candidates is uncertain. The Hyades stars we have observed and the three Pleiades very low mass stars are the lowest mass members of any open cluster whose membership has been confirmed by radial velocities and whose chromospheric activity has been measured. We see no change in chromospheric activity at the boundary where stars are expected to become fully convective (M approximately equals 0.3 solar mass) in either cluster. In the Pleiades, however, there may be a decrease in chromospheric activity for stars with (V-I)K greater than 3.5 (M less than or equal to 0.1 solar mass). Publication: The Astronomical Journal Pub Date: July 1994 DOI: 10.1086/117054 Bibcode: 1994AJ....108..160S Keywords: Astronomical Spectroscopy; Dwarf Stars; Radial Velocity; Star Clusters; Stellar Spectra; Charge Coupled Devices; Pleiades Cluster; Stellar Mass; Astrophysics; OPEN CLUSTERS AND ASSOCIATIONS: INDIVIDUAL: HYADES; OPEN CLUSTERS AND ASSOCIATIONS: INDIVIDUAL: PLEIADES; STARS: BROWN DWARFS full text sources ADS | data products SIMBAD (33)

We present an analysis of deep Hubble Space Telescope (HST)/Wide Field Camera 3 near-IR (NIR) imaging data of the globular cluster (GC) M4. The best-photometry NIR color–magnitude diagram (CMD) clearly shows the main sequence extending toward the expected end of the hydrogen-burning limit and going beyond this point toward fainter sources. The white dwarf (WD) sequence can be identified. As such, this is the deepest NIR CMD of a GC to date. Archival HST optical data were used for proper-motion cleaning of the CMD and for distinguishing the WDs from brown dwarf (BD) candidates. Detection limits in the NIR are around F110W ≈ 26.5 mag and F160W ≈ 27 mag, and in the optical around F775W ≈ 28 mag. Comparing our observed CMDs with theoretical models, we conclude that we have reached beyond the H-burning limit in our NIR CMD and are probably just above or around this limit in our optical–NIR CMDs. Thus, any faint NIR sources that have no optical counterpart are potential BD candidates, since the optical data are not deep enough to detect them. We visually inspected the positions of NIR sources that are fainter than the H-burning limit in F110W and for which the optical photometry did not return a counterpart. We found in total five sources for which we did not get an optical measurement. For four of these five sources, a faint optical counterpart could be visually identified, and an upper optical magnitude was estimated. Based on these upper optical magnitude limits, we conclude that one source is likely a WD, one source could be either a WD or BD candidate, and the remaining two sources agree with being BD candidates. No optical counterpart could be detected for just one source, which makes this source a good BD candidate. We conclude that we found in total four good BD candidates.

Brown dwarfs are failed stars with very low mass (13–75 Jupiter mass) and an effective temperature lower than 2 500 K. Their mass range is between Jupiter and red dwarfs. Thus, they play a key role in understanding the gap in the mass function between stars and planets. However, due to their faint nature, previous searches are inevitably limited to the solar neighbourhood (20 pc). To improve our knowledge of the low mass part of the initial stellar mass function and the star formation history of the Milky Way, it is crucial to find more distant brown dwarfs. Using James Webb Space Telescope (JWST) COSMOS-Web data, this study seeks to enhance our comprehension of the physical characteristics of brown dwarfs situated at a distance of kpc scale. The exceptional sensitivity of the JWST enables the detection of brown dwarfs that are up to 100 times more distant than those discovered in the earlier all-sky infrared surveys. The large area coverage of the JWST COSMOS-Web survey allows us to find more distant brown dwarfs than earlier JWST studies with smaller area coverages. To capture prominent water absorption features around 2.7 ${\unicode{x03BC}}$ m, we apply two colour criteria, $\text{F115W}-\text{F277W}+1\lt\text{F277W}-\text{F444W}$ and $\text{F277W}-\text{F444W}\gt\,0.9$ . We then select point sources by CLASS_STAR, FLUX_RADIUS, and SPREAD_MODEL criteria. Faint sources are visually checked to exclude possibly extended sources. We conduct SED fitting and MCMC simulations to determine their physical properties and associated uncertainties. Our search reveals 25 T-dwarf candidates and 2 Y-dwarf candidates, more than any previous JWST brown dwarf searches. They are located from 0.3 to 4 kpc away from the Earth. The spatial number density of 900–1 050 K dwarf is $(2.0\pm0.9) \times10^{-6}\text{ pc}^{-3}$ , 1 050–1 200 K dwarf is $(1.2\pm0.7) \times10^{-6}\text{ pc}^{-3}$ , and 1 200–1 350 K dwarf is $(4.4\pm1.3) \times10^{-6}\text{ pc}^{-3}$ . The cumulative number count of our brown dwarf candidates is consistent with the prediction from a standard double exponential model. Three of our brown dwarf candidates were detected by HST, with transverse velocities $12\pm5$ , $12\pm4$ , and $17\pm6$ km s $^{-1}$ . Along with earlier studies, the JWST has opened a new window of brown dwarf research in the Milky Way thick disk and halo.

We present the results of a deep CCD-based IZ photometric survey of a ~ 1 deg2 area in the central region of the Pleiades Galactic open cluster. The magnitude coverage of our survey (from down to 22) allows us to detect substellar candidates with masses between 0.075 and 0.03 . Details of the photometric reduction and selection criteria are given. Finder charts prepared from the I-band images are provided.

view Abstract Citations (49) References (41) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Lithuim in Very Low-Mass Stars in the Pleiades Oppenheimer, B. R. ; Basri, G. ; Nakajima, T. ; Kulkarni, S. R. Abstract High-resolution, Keck Telescope echelle observations from 630 nm to 850 nm of seven Pleiads with spectral types from M5 to M6.5 reveal rather rapid rotation, with an average v sin i ~ 52 km s(-1) , and chromospheric activity in Hα emission. The activity in these stars is not any stronger than that of other Pleiades low-mass stars, despite the expected high contrast of Hα with their cool photospheres and their rapid rotation. This shows that the ``levelling off'' of Hα equivalent widths previously noted in low-mass stars in young clusters is not related to the conventional rotation-activity connection. None of the stars previously categorized as brown dwarf candidates have lithium signatures in their spectra. They are, therefore, very low-mass stars and not brown dwarfs. However, two stars, HHJ 339 and HHJ 430, 1 and 2 magnitudes above the Pleiades zero-age main sequence, do show absorption due to Li 1 at 670.8 nm and in the subordinate feature at 812.6 nm. These two stars are also rotating very rapidly. These facts strongly suggest that these stars are rather young. Their proper motions and radial velocities agree with those measured for the Pleiades as a whole. We discuss various explanations for these stars, none of which is completely satisfactory. In one scenario they represent very late star formation in the Pleiades cluster (implying a huge range in the ages of Pleiads). This seems unpalatable given the lack of matter dense enough to form stars in the Pleiades at present. Another possibility is that these stars formed in a nearby, more recent star formation site and drifted into the Pleiades. Although the cluster recently passed through a clump of young Taurus stars, we do not see how it could ``accrete'' two of them. In our most feasible explanation, we posit that a cloud which was a member of the ``Pleiades Supercluster'' recently formed stars, which are now scattered between us and the Pleiades. HHJ 339 and HHJ 430 could be members of this group whose motion has now brought them near the older open star cluster. Publication: The Astronomical Journal Pub Date: January 1997 DOI: 10.1086/118252 Bibcode: 1997AJ....113..296O full text sources ADS | data products SIMBAD (12)

We report proper motion measurements for 25 very-low mass (VLM) star and brown dwarf (BD) candidates of the Ple iades cluster previously identified by Bouvier et al. (1998). Proper motions are measured with an accuracy of 9 mas/yr, compared to an expected tangential motion of about 50 mas/yr for Pleiades members. Of the 25 candidates, 15 have a membership probability of 95% or more and 7 are rejected as being field dwarfs. The 3 remaining candidates exhibit independent evidence for membership (lithium absorption or long-term proper motion). From the firm identification of Pleiades VLM and BD members, the cluster's substellar mass function is revised to dN/dM ∝ M^(-0.5) in the mass range from 0.04 to 0.3 M_⊙.

We have obtained deep charge coupled device (CCD)V and I images of a number of fields near the center of the Pleiades open cluster. We have also obtained imaging data for Praesepe, a very similar cluster in terms of distance and richness but nearly 10 times older than the Pleiades. Because brown dwarfs are predicted to become much fainter and cooler between Pleiades and Praesepe ages, this provides a powerful differential technique for placing constraints on the brown dwarf population in open clusters. Combined with our previously reported observations, we now have data for about 0.4 sq deg in the Pleiades, corresponding roughly to 5% of the area of that cluster. We have searched the new CCD frames for additional Pleiades brown dwarf candidates. Two possible candidates are present, the faintest of which has V approximately equal to 22.5, (V-I)(sub K) approximately equal to 4.6. Because we do not have proper motion data and the colors of these objects are not redder than the reddest known field stars, it is possible that some or all of our candidates are somewhat higher mass field stars rather than Pleiades-age brown dwarfs. Even if all six of the proposed brown dwarf candidates in our 0.4 sq deg field are Pleiades members, the relatively small number found suggests that low mass stars or brown dwarfs do not contribute significantly to the total mass of the cluster.

We present an analysis of the second epoch Hubble Space TelescopeWide Field Camera 3 F110W near-infrared (NIR) imaging data of the globular cluster M 4. The new data set suggests that one of the previously suggested four brown dwarf candidates in this cluster is indeed a high-probability cluster member. The position of this object in the NIR colour–magnitude diagrams (CMDs) is in the white dwarf/brown dwarf area. The source is too faint to be a low-mass main-sequence (MS) star, but, according to theoretical considerations, also most likely somewhat too bright to be a bona-fide brown dwarf. Since we know that the source is a cluster member, we determined a new optical magnitude estimate at the position the source should have in the optical image. This new estimate places the source closer to the white dwarf sequence in the optical–NIR CMD and suggests that it might be a very cool (Teff ≤ 4500 K) white dwarf at the bottom of the white dwarf cooling sequence in M 4, or a white dwarf/brown dwarf binary. We cannot entirely exclude the possibility that the source is a very massive, bright brown dwarf, or a very low-mass MS star, however, we conclude that we still have not convincingly detected a brown dwarf in a globular cluster, but we expect to be very close to the start of the brown dwarf cooling sequence in this cluster. We also note that the MS ends at F110W ≈ 22.5 mag in the proper-motion cleaned CMDs, where completeness is still high.

Context.Although more than 2000 brown dwarfs have been detected to date, mainly from direct imaging, their characterisation is difficult due to their faintness and model-dependent results. In the case of transiting brown dwarfs, however, it is possible to make direct high-precision observations.Aims.Our aim is to investigate the nature and formation of brown dwarfs by adding a new well-characterised object, in terms of its mass, radius and bulk density, to the currently small sample of less than 20 transiting brown dwarfs.Methods.One brown dwarf candidate was found by the KESPRINT consortium when searching for exoplanets in the K2 space mission Campaign 16 field. We combined the K2 photometric data with a series of multicolour photometric observations, imaging, and radial velocity measurements to rule out false positive scenarios and to determine the fundamental properties of the system.Results.We report the discovery and characterisation of a transiting brown dwarf in a 5.17-day eccentric orbit around the slightly evolved F7 V star EPIC 212036875. We find a stellar mass of 1.15 ± 0.08M⊙, a stellar radius of 1.41 ± 0.05R⊙, and an age of 5.1 ± 0.9 Gyr. The mass and radius of the companion brown dwarf are 51 ± 2MJand 0.83 ± 0.03RJ, respectively, corresponding to a mean density of 108−13+15g cm−3.Conclusions.EPIC 212036875 b is a rare object that resides in the brown-dwarf desert. In the mass-density diagram for planets, brown dwarfs, and stars, we find that all giant planets and brown dwarfs follow the same trend from ~0.3MJto the turn-over to hydrogen burning stars at ~ 73MJ. EPIC 212036875 b falls close to the theoretical model for mature H/He dominated objects in this diagram as determined by interior structure models. We argue that EPIC 212036875 b formed via gravitational disc instabilities in the outer part of the disc, followed by a quick migration. Orbital tidal circularisation may have started early in its history for a brief period when the brown dwarf’s radius was larger. The lack of spin–orbit synchronisation points to a weak stellar dissipation parameter (Q⋆′≳ 108), which implies a circularisation timescale of ≳23 Gyr, or suggests an interaction between the magnetic and tidal forces of the star and the brown dwarf.

Context. Studying the accretion process in very low-mass objects has important implications for understanding their formation mechanism. Many nearby late-M dwarfs that have previously been identified in the field are in fact young brown dwarf members of nearby young associations. Some of them are still accreting. They are therefore excellent targets for further studies of the accretion process in the very low-mass regime at different stages. Aims. We aim to search for accreting young brown dwarf candidates in a sample of 85 nearby late-M dwarfs. Methods. Using photometric data from DENIS, 2MASS, and WISE, we constructed the spectral energy distribution of the late- M dwarfs based on BT-Settl models to detect infrared excesses. We then searched for lithium and Hα emission in candidates that exhibit infrared excesses to confirm their youth and the presence of accretion. Results. Among the 85 late-M dwarfs, only DENIS-P J1538317−103850 (M5.5) shows strong infrared excesses in WISE bands. The detection of lithium absorption in the M5.5 dwarf and its Gaia trigonometric parallax indicate an age of ~1 Myr and a mass of 47 MJ. The Hα emission line in the brown dwarf shows significant variability that indicates sporadic accretion. This 1 Myr-old brown dwarf also exhibits intense accretion bursts with accretion rates of up to 10−7.9 M⊙ yr−1. Conclusions. Our detection of sporadic accretion in one of the youngest brown dwarfs might imply that sporadic accretion at early stages could play an important role in the formation of brown dwarfs. Very low-mass cores would not be able to accrete enough material to become stars, and thus they end up as brown dwarfs.

view Abstract Citations (255) References (42) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Lithium in Brown Dwarf Candidates: The Mass and Age of the Faintest Pleiades Stars Basri, Gibor ; Marcy, Geoffrey W. ; Graham, James R. Abstract We present high-resolution optical spectroscopy and infrared photometry of one of the lowest luminosity Pleiades stars, PPL 15. Its cluster membership is strengthened by both its measured radial velocity and Hα strength. Its reported mass is 0.06 Msun, based on its I-band luminosity and the Pleiades age of 75 Myr as reported by Stauffer, Hamilton, & Probst in 1994. We confirm its luminosity with JHK photometry. Such a low mass for PPL 15 implies that it should currently retain lithium, unlike all low-mass Pleiades stars tested so far. Our Keck HIRES spectrum of PPL 15 indeed exhibits the lithium absorption feature with an equivalent width of 0.5 Å. We estimate the likelihood this detection is spurious to be less than 1%. Thus, PPL 15 passes the lithium test for brown dwarf status. Calculations of the luminosity as a function of mass and age for very low mass stars, along with the history of lithium depletion, have been provided by Nelson, Rappaport, & Chiang in 1993. Lithium is depleted in HHJ 3, which is only a little brighter than PPL 15. The self-consistent interpretation with both observations and theory is that the age of the Pleiades is ∼115 Myr. If so, the derived mass for PPL 15 increases to ∼0.078 Msun. The canonical 75 Myr age was derived from the upper main-sequence turnoff, but it substantially increases if core convective overshoot is included. Such mixing could bring the two methods of age determination into agreement. It is therefore possible that the ages of young clusters have generally been underestimated. The luminosity of brown dwarfs in these clusters would thus have been overestimated. Publication: The Astrophysical Journal Pub Date: February 1996 DOI: 10.1086/176842 Bibcode: 1996ApJ...458..600B Keywords: GALAXY: OPEN CLUSTERS AND ASSOCIATIONS: INDIVIDUAL NAME: PLEIADES; STARS: ABUNDANCES; STARS: EVOLUTION; STARS: LOW-MASS; BROWN DWARFS full text sources ADS | data products SIMBAD (9)

The fundamental properties of brown dwarfs evolve with age. Models describing the evolution of luminosities and effective temperatures, among other physical parameters, can be empirically constrained using brown dwarfs of various masses in star clusters of well determined age and metallicity. We aim to carry out a spectroscopic and photometric characterization of low-mass brown dwarfs of the ~120 Myr old Pleiades open cluster. We obtained low-resolution near-infrared spectra of the J=17.4-18.8 mag candidate L-type brown dwarfs PLIZ 28 and 35, BRB 17, 21, 23, and 29, which are Pleiades members by photometry and proper motion. We also obtained spectra of the well-known J=15.4-16.1 mag late M-type cluster members PPl 1, Teide 1, and Calar 3. We find that the former six objects have early- to mid-L spectral types and confirm previously reported M-types for the other three objects. The spectra of the L0-type BRB 17 and PLIZ 28 present a triangular H-band continuum shape, indicating that this peculiar spectral feature persists until at least the age of the Pleiades. We add to our sample 36 reported M5-L0-type cluster members, collecting their I_c - and UKIDSS ZYJHK-band photometry. We confirm a possible interleaving of the Pleiades and field L-type sequences in the JHK absolute magnitude versus spectral type diagrams, and quantify marginally redder Pleiades J-K colours, by 0.12+-0.20 mag, possibly related to both reddening and youth. Using field dwarf bolometric correction - and effective temperature - spectral type relations, we obtain the Hertzsprung-Russell diagram of the Pleiades sample. Theoretical models reproduce well the spectral sequence at M5.5-9, but appear to overestimate the luminosity or underestimate the effective temperature at L0-5. For the Pleiades early- to mid L-type brown dwarfs, we estimate theoretical masses to be in the range 0.025-0.035 M_Sol.