Spectroscopy of New Substellar Candidates in the Pleiades: Toward a Spectral Sequence for Young Brown Dwarfs
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We present optical and near-IR spectroscopy (600--1000 nm) of 8 faint (I$>$18) very red (R--I$>$2.2) objects discovered in a deep CCD survey of the Pleiades (Zapatero-Osorio et al. 1996). We compare them with reliable cluster members like PPl 15 and Teide 1, and with several field very late-type dwarfs (M4--M9.5), which were observed with similar instrumental configurations. Using pseudocontinuum ratios we classify the new substellar candidates in a spectal sequence defined with reference to field stars of known spectral types. We also reclassify PPl 15 and Teide 1 in a self-consistent way. The likelihood of membership for the new candidates is assesed via the study of their photospheric features, Ha emission, radial velocity, and consistency of their spectral types and I magnitudes with known cluster members. Four of the new substellar candidates are as late or later than PPl~15 (M6.5), but only one, Calar 3 (M8), clearly meets all our membership criteria. It is indeed an object very similar to the brown dwarf Teide 1. Calar 3, together with Teide 1, allows one to compare the spectroscopic characteristics of Pleiades brown dwarfs with those of old very cool dwarfs. The overall spectral properties are similar, but there are slight differences in the NaI doublet (818.3 nm, 819.5 nm), VO molecular band (740 nm), and some spectral ratios, which are probably related to lower surface gravity in the young Pleiades brown dwarfs than in field stars. Finally, we propose a way of improving future CCD-based brown dwarf surveys by using narrow-band near-IR pseudocontinuum filters.Keywords:
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Abstract We present results of an optical and near‐infrared (IR) 1.8 deg 2 survey in the Pleiades open cluster to search for substellar objects. From optical I ‐band images from the CFHT and J ‐band images from the 3.5m CAHA Telescope, we identify 18 faint and very red L brown dwarf candidates, with I > 20.9 and I – J > 3.2. The follow‐up observations of nine objects in the H ‐ and K s ‐bands confirm that eight belong to the IR sequence of the cluster and the proper motion measurements of seven candidates confirm that they are Pleiades members. A preliminary estimation of the substellar mass spectrum d N/ d M in the form of a power law M c – α provides α = +0.57 ± 0.14. We extrapolate this function to estimate the number of planetary mass objects that could be present in the cluster down to 1 M Jup . Sensitive searches combining far red and near‐IR observations may unveal these objects in a near future. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
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Aims.We present a search for low-mass brown dwarfs in the Pleiades open cluster. The identification of Pleiades members fainter and cooler than those currently known allows us to constrain evolutionary models for L dwarfs and to extend the study of the cluster mass function to lower masses.
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COSMOS measurements of R and I Schmidt plates are used to determine the luminosity function and hence mass function of the Pleiades open cluster. Star counts are made in the cluster and the field star contribution, measured outside the cluster, is subtracted. The mass function is found down to 0.06 m⊙. A lower limit of 30 brown dwarfs is found; the mass function is flat at the lowest masses. The above are based on a Pleiades age of 63 Myr and theoretical models of D’Antona & Mazzitelli.
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Initial mass function
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Mass segregation
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We present optical and near-IR spectroscopy (600--1000 nm) of 8 faint (I$>$18) very red (R--I$>$2.2) objects discovered in a deep CCD survey of the Pleiades (Zapatero-Osorio et al. 1996). We compare them with reliable cluster members like PPl 15 and Teide 1, and with several field very late-type dwarfs (M4--M9.5), which were observed with similar instrumental configurations. Using pseudocontinuum ratios we classify the new substellar candidates in a spectal sequence defined with reference to field stars of known spectral types. We also reclassify PPl 15 and Teide 1 in a self-consistent way. The likelihood of membership for the new candidates is assesed via the study of their photospheric features, Ha emission, radial velocity, and consistency of their spectral types and I magnitudes with known cluster members. Four of the new substellar candidates are as late or later than PPl~15 (M6.5), but only one, Calar 3 (M8), clearly meets all our membership criteria. It is indeed an object very similar to the brown dwarf Teide 1. Calar 3, together with Teide 1, allows one to compare the spectroscopic characteristics of Pleiades brown dwarfs with those of old very cool dwarfs. The overall spectral properties are similar, but there are slight differences in the NaI doublet (818.3 nm, 819.5 nm), VO molecular band (740 nm), and some spectral ratios, which are probably related to lower surface gravity in the young Pleiades brown dwarfs than in field stars. Finally, we propose a way of improving future CCD-based brown dwarf surveys by using narrow-band near-IR pseudocontinuum filters.
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This paper presents the results of a deep infrared survey for low-mass brown dwarfs in the Pleiades cluster. By comparing J-band (1.25 μm) and I-band (0.83 μm) photometry of an ∼450 arcmin2 area with the latest theoretical models of pre-main-sequence substellar evolution, a number of candidates were identified with J-band magnitudes and I - J color limits consistent with Pleiades membership. The mean 10 σ completeness limits of J = 19.25 and I = 23.25 mag correspond to an age-dependent (80–125 Myr) effective depth of (23–28)MJ at which Pleiades members of late L spectral types and temperatures near ∼1650 K (I - J color ∼4 mag) are predicted to begin making a transition to methane-dominated spectra. Of the preliminary candidates, three are more likely field stars, one is a previously known Pleiades brown dwarf (BPL 81), and the remaining are new substellar candidates, all ranging in membership-dependent mass (again for 80–125 Myr ages) from (22–26)MJ to (46–57)MJ. Follow-up infrared photometry and higher resolution image profile analysis with the Keck telescopes have eliminated all except about five new substellar Pleiades candidates as likely cluster members. Additional follow-up observations including proper-motion and spectral analysis, as well as more comprehensive multiband imaging, will be required to clarify membership likelihood for these Pleiades candidates. Any of the remaining new substellar candidates presented herein, if confirmed as cluster members, would be the lowest mass brown dwarfs discovered in the Pleiades by any survey to date.
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We report on the results of the spectroscopy of 10 objects previously classified as brown dwarf candidates via RIJHK colors by Eisenbeiss et al. (2009), who performed deep imaging observations on a 0.4 sq.deg. field at the edge of the Pleiades. We describe and judge on classification techniques in the region of M-type stars. To classify and characterise the objects, visual and near infrared spectra have been obtained with VLT FORS and ISAAC. The spectral classification was performed using the shape of the spectra as well as spectral indices that are sensitive to the spectral type and luminosity class of Mtype stars and late M-type brown dwarfs. Furthermore a spectrophotometric distance was calculated and compared the distance of the Pleiades to investigate the membership probability. As a second argument we analyzed the proper motion. The brown dwarf candidates were found not to be brown dwarfs, but late-K to mid-M-type dwarf stars. Based on the obtained distance and tabulated proper motions we conclude that all objects are background dwarf stars.
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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. ([CITE]). 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 in the mass range from 0.04 to 0.3 .
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Using new infrared data we have reassessed the membership status of candidate low-mass Pleiads unearthed by the International Telescope Project I Z survey.Those with I-K colours consistent with membership of the Pleiades have been compiled with candidate brown dwarfs identified by three other large, deep far-red CCD surveys of the cluster to yield the biggest magnitude-limited sample of substellar members to date.We fit King profiles to their spatial distribution to determine the Pleiades brown dwarf core radius to be r c = 2.22 +1.36 -0.67 degrees (or 5.0 +3.0 -1.5 pc).This is consistent with a continuation of the r c ∝ m -0.5 relationship found previously for the higher-mass stellar members and suggests that the brown dwarf members are also dynamically relaxed.Using our spatial model we derive the Pleiades mass function in the substellar regime and are able to place stringent limits on its shape.We find that it is well represented by a power law with index α = 0.41 ± 0.08 (0.035 M M 0.3 M ).This result is largely insensitive to our choice of evolutionary model and uncertainties in the cluster age and distance.It is only marginally sensitive to the brown dwarf binary fraction.By assuming that the cluster mass function continues to rise down to the deuterium-burning limit, we estimate that the total brown dwarf mass of the Pleiades is 13 +4 -3 M .This only represents ∼2 per cent of the total cluster mass.Given that the present-day cluster mass function should be a good representation of the initial mass function, we conclude that brown dwarfs do not contribute significantly to disc dark matter.
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The brown dwarf population in the Pleiades cluster has been probed in a deep 850 arcmin2 RIJK survey. The survey is complete to I=21.4 in 76% of the area and to I=20.2 in the remaining 24%. Photometry of 32 previously known members is presented together with 8 new candidates, four of which are below the brown dwarf limit. The faintest one is the lowest mass brown dwarf candidate found hitherto in the Pleiades (I=20.55, 0.04Msun). The derived Pleiades luminosity function is compared to the most recent theoretical mass-luminosity relations and is consistent with a power-law index in the mass function between 0 and 1 to the limit of this survey.
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We have obtained low-resolution optical spectra of three faint brown dwarf candidates in the Pleiades open cluster. The objects observed are Roque 12 (I=18.5), Roque 5 (I=19.7) and Roque 25 (I=21.2). The spectrum of Roque 25 does not show the strong TiO bandheads that characterize the optical spectra of M-type stars, but molecular bands of CaH, CrH and VO are clearly present. We classify Roque 25 as an early L-type brown dwarf. Using current theoretical evolutionary tracks we estimate that the transition from M-type to L-type in the Pleiades (age ~ 120 Myr) takes place at Teff ~ 2200 K or M ~ 0.04 Msol. Roque 25 is a benchmark brown dwarf in the Pleiades because it is the first known one that belongs to the L-type class. It provides evidence that the IMF extends down to about 0.035 Msol, and serves as a guide for future deep searches for even less massive young brown dwarfs.
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