arXiv subjects

E. L. Martin

Publications and source records attributed to E. L. Martin.

At least 163 records · Page 9Linked to original sources

arXiv2000

DENIS-P J104814.7-395606.1: An M9 dwarfs at about 4 pc

We present the discovery of a previously unknown member of the immediate solar neighbourhood, DENIS-P J104814.7-395606.1 (hereafter DENIS 1048-39), identified while mining the DENIS database for new nearby stars. A HIRES echelle spectrum obtained with the 10-m Keck telescope shows that it is an M9 dwarf. DENIS 1048-39 has a very bright apparent magnitude (I=12.67) for its spectral type and colour (I-J=3.07), and is therefore very nearby. If it is single its distance is only 4.1 +- 0.6pc, ranking it as between our twelfth and fortyth closest neighbour. It is also the closest star or brown dwarf with a spectral type later than M7V. Its proper motion was determined through comparison of Sky atlas Schmidt plates, scanned by the MAMA microdensitometer, with the DENIS images. At 1.53"/yr it further attests the closeness of DENIS 1048-39, and hence its dwarf status. These characteristics make it an obvious target for further detailed studies.

astro-ph↗

arXiv2000

A search for Jovian-mass planets around CM Draconis using eclipse minima timing

For the eclipsing binary system CM Draconis, eclipse minimum times have been monitored with high precision between 1994 and 1999. Periodic deviations of minimum times from a linear ephemeris may indicate the presence of an orbiting third body. Individual measurements of 41 eclipse minimum times result in a standard deviation from linear ephemeris of 5.74 seconds. A power spectral analysis of the residuals reveals only one periodicity with more then 2 seconds amplitude. This feature, with a periodicity between 750 and 1050 days has an amplitude of 2.8+-0.5 seconds, and is also present with similar phases if the power spectral analysis is performed independently for primary and secondary eclipses. It would be compatible with a planet of 1.5-3 Jupiter masses at an orbital distance of 1.1-1.45 AU to the binary barycenter. The assignation of a planet to the CM Dra system can however only be upheld if this periodicity can be followed in future observations for several years. For low-mass eclipsing binary stars, the method of eclipse minimum timing allows one to reach mass limits for the detection of third bodies well below that feasible by radial velocity measurements.

astro-ph↗

arXiv2000

Membership and Multiplicity among Very Low-Mass Stars and Brown Dwarfs in the Pleiades Cluster

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. Eight objects out of 45 appear to be non-members. A search for companions among 34 very low-mass Pleiades members (M$\le$0.09 M$_\odot$) in high-spatial resolution images obtained with the Hubble Space Telescope and the adaptive optics system of the Canada-France-Hawaii telescope produced no resolved binaries with separations larger than 0.2 arcsec (a ~ 27 AU; P ~ 444 years). Nevertheless, we find evidence for a binary sequence in the color-magnitude diagrams, in agreement with the results of Steele & Jameson (1995) for higher mass stars. 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 (Duquennoy & Mayor 1991). 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 non-members found in this work. We find a contamination level of 33% among the brown dwarf candidates identified by Bouvier et al. (1998). 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.

astro-ph↗

arXiv2000

Ages, Distances, and the Initial Mass Functions of Stellar Clusters

We provide a review of the current status of several topics on the ages, distances, and mass functions of open clusters, with a particular emphasis on illuminating the areas of uncertainty. Hipparcos has obtained parallaxes for nearby open clusters that have expected accuracies much better than has been previously achievable. By using the lithium depletion boundary method and isochrone fitting based on much improved new theoretical evolutionary models for low mass stars, it is arguable that we will soon have have much better age scales for clusters and star-forming regions. With improved optical and near-IR cameras, we are just now beginning to extend the mass function of open clusters like the Pleiades into the regime below the hydrogen burning mass limit. Meanwhile, observations in star-forming regions are in principle capable of identifying objects down to of order 10 Jupiter masses.

astro-ph↗

arXiv1999

An L-type substellar object in Orion: reaching the mass boundary between brown dwarfs and giant planets

We present J-band photometry and low-resolution optical spectroscopy (600-1000 nm) for one of the faintest substellar member candidates in the young sigma Ori cluster, SOri 47 (I=20.53, Bejar et al. 1999). Its very red (I-J)=3.3+/-0.1 color and its optical spectrum allow us to classify SOri 47 as an L1.5-type object which fits the low-luminosity end of the cluster photometric and spectroscopic sequences. It also displays atmospheric features indicative of low gravity such as weak alkaline lines and hydride and oxide bands, consistent with the expectation for a very young object still undergoing gravitational collapse. Our data lead us to conclude that SOri 47 is a true substellar member of the sigma Ori cluster. Additionally, we present the detection of LiI in its atmosphere which provides an independent confirmation of youth and substellarity. Using current theoretical evolutionary tracks and adopting an age interval of 1-5 Myr for the sigma Ori cluster, we estimate the mass of SOri 47 at 0.015+/-0.005 Msun, i.e. at the minimum mass for deuterium burning, which has been proposed as a definition for the boundary between brown dwarfs and giant planets. SOri 47 could well be the result of a natural extension of the process of cloud fragmentation down to the deuterium burning mass limit; a less likely alternative is that it has originated from a protoplanetary disc around a more massive cluster member and later ejected from its orbit due to interacting effects within this rather sparse (~12 objects/pc^3) young cluster.

astro-ph↗

arXiv1999

Mass Function at the end of the Main Sequence: the M35 Open Cluster

We present very deep RIc photometry of the M35 open cluster. We have covered 30' \times 30', equivalent to about a fourth of the total area of the cluster. The data range from I_c=17.5 to 23.5 magnitudes, and the color interval is $0.5 \le (R-I)_c \le 2.5$. Roughly, these values correspond from $0.6 M_\odot$ to $0.1 M_\odot$ in the cases of members of the cluster. By using the location of the stars on a Color-Magnitude Diagram, we have selected possible members of this cluster and obtained the Luminosity and Mass Functions of the M35 open cluster and compared them with those from the Pleiades.

astro-ph↗

arXiv1999

Radio emission from RASS sources south of Taurus-Auriga

We present a 8.4 GHz VLA survey of 50 optical counterparts to 46 ROSAT All-Sky survey (RASS) X-ray sources south of the Taurus-Auriga star-forming region. This survey detected 3 sources with a sensitivity limit of ~ 0.12 mJy. Merging our sample with other radio observations of sources in and around Taurus-Auriga, we find scarce radio emission among RASS sources south of the Taurus-Auriga molecular clouds. Our data support the evidence that these sources are older than weak-lined T Tauri stars, most of them probably being close to the ZAMS.

astro-ph↗

arXiv1998

Discovery of a Very Low-Mass Binary with HST/NICMOS

Hubble Space Telescope NICMOS observations are presented of six brown dwarf candidates in the Pleiades open cluster. One of them, namely CFHT-Pl-18, is clearly resolved as a binary with an angular separation of 0".33. The very low density of contaminating background stars in our images and the photometry of the components support that this system is a physical binary rather than a chance projection. All the available photometric and spectroscopic data indicate that the CFHT-Pl-18 system is likely a member of the Pleiades cluster, but a final confirmation will have to wait until lithium can be detected. Assuming cluster membership, we compare our NICMOS photometry with evolutionary models, and find that the inclusion of the effects of dust grains is necessary for fitting the data. We estimate that the masses of the components are about 0.045 Msol and 0.035 Msol. The binary system has a projected separation of 42 AU (for a distance of 125 pc) that is common among stellar binaries.

astro-ph↗

arXiv1998

Brown Dwarfs in the Pleiades Cluster. III. A deep IZ survey

We present the results of a deep CCD-based IZ photometric survey of a ~1 sq. deg area in the central region of the Pleiades Galactic open cluster. The magnitude coverage of our survey (from I~17.5 down to 22) allows us to detect substellar candidates with masses between 0.075 and 0.03 Msol. Details of the photometric reduction and selection criteria are given. Finder charts prepared from the I-band images are provided.

astro-ph↗

arXiv1998

The First L-type Brown Dwarf in the Pleiades

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.

astro-ph↗

arXiv1998

Near-Term Detectability of Terrestrial Extrasolar Planets: TEP Network Observations of CM Draconis

Results from a photometric search for extrasolar planetary transits across the eclipsing binary CM Dra are presented. The TEP (Transits of Extrasolar Planets) network has observed this star since 1994, and a lightcurve with 617 hours of coverage has been obtained. The data give a complete phase coverage of the CM Dra system at each of the 3 years of observations, with a noise of less than 5 mmag. New epoch and period values for CM Dra are derived, and a low flare rate of 0.025/hr has been confirmed. The absence of periodic variations in eclipse minimum times excludes the presence of very massive planets with periods of less than a few years. The lightcurve was visually scanned for the presence of unusual events which may be indicative of transits of extrasolar planets with 'massive earth' sizes. Six suspicious events were found which are being followed up for future transits, by planets with sizes between 1.5 and 2.5 RE (Earth Radii). However, none of these events has amplitudes compatible with planets larger than 2.5 RE. Coplanar planets larger than 2.5 RE and with orbital periods of less than 60 days can therefore be ruled out with a confidence of about 80%. Planets smaller than 1.5 RE cannot be detected in the data without a sub-noise detection algorithm. A preliminary signal detection analysis shows that there is a 50% detection confidence for 2 RE planets with a period from 10 to 30 days with the current data. This data-set demonstrates that it is possible to detect terrestrial sized planets with ground based photometry, and that strong constraints on the sizes of planets orbiting in the plane of the CM Dra system can be set.

astro-ph↗

arXiv1998

A New Pleiades Member at the Lithium Substellar Boundary

We present the discovery of an object in the Pleiades open cluster, named Teide 2, with optical and infrared photometry which place it on the cluster sequence slightly below the expected substellar mass limit. We have obtained low- and high-resolution spectra that allow us to determine its spectral type (M6), radial velocity and rotational broadening; and to detect H$_α$ in emission and Li I 670.8 nm in absorption. All the observed properties strongly support the membership of Teide 2 into the Pleiades. This object has an important role in defining the reappearance of lithium below the substellar limit in the Pleiades. The age of the Pleiades very low-mass members based on their luminosities and absence or presence of lithium is constrained to be in the range 100--120 Myr.

physics.space-ph↗

arXiv1998

A brown dwarf candidate in the Praesepe Open Cluster

We present optical and infrared observations of RPr1, a faint (I = 21.01) and very red object (I - K = 4.57) discovered in a deep CCD survey, covering an area of 800 square arcmin of the Praesepe open cluster. A low resolution spectrum shows that RPr1 is a very late object, the latest object in Praesepe for which a spectrum has been taken to date. Our estimates give a mass between 0.063 and 0.084 solar masses, and indicate that RPr1 may turn out to be the first brown dwarf in this cluster.

astro-ph↗

arXiv1997

New Brown Dwarfs in the Pleiades Cluster

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.

astro-ph↗

arXiv1997

Library of high-resolution UES echelle spectra of F, G, K and M field dwarf stars

We present a library of Utrecht echelle spectrograph (UES) observations of a sample of F, G, K and M field dwarf stars covering the spectral range from 4800 AA to 10600 AA with a resolution of 55000. These spectra include some of the spectral lines most widely used as optical and near-infrared indicators of chromospheric activity such as H_beta, Mg I b triplet, Na I D_1, D_2, He I D_3, H_alpha, and Ca II IRT lines, as well as a large number of photospheric lines which can also be affected by chromospheric activity. The spectra have been compiled with the aim of providing a set of standards observed at high-resolution to be used in the application of the spectral subtraction technique to obtain the active-chromosphere contribution to these lines in chromospherically active single and binary stars. This library can also be used for spectral classification purposes. A digital version with all the spectra is available via ftp and the World Wide Web (WWW) in both ASCII and FITS formats.

astro-ph↗

arXiv1997

On a Rapid Lithium Enrichment and Depletion of K Giant Stars

A model scenario has recently been introduced to explain the presence of very strong Li lines in the spectra of some low mass K giant stars (de la Reza et al. 1996). In this scenario all ordinary, Li poor, K giants become Li rich during a short time ($\sim 10^5 yr$) when compared to the red giant phase of 5 10^7 yr. In this ``Li period'', a large part of the stars are associated with an expanding thin circumstellar shell supposedly triggered by an abrupt internal mixing mechanism resulting in a surface new ^7Li enrichment. This letter presents near 40 Li rich K giants known up to now. The distribution of these Li rich giants, along with other 41 observed K giants that have shell, but are not Li rich, in a color-color IRAS diagram confirms this scenario, indicating, also as a new result, that a rapid Li depletion takes place on a time scale of between $\sim 10^3$ and 10^5 yr. This model explains the problem of the presence of K giants with far infrared excesses presented by Zuckerman et al. (1995). Other present and future tests of this scenario are briefly discussed.

astro-ph↗

arXiv1997

Brown Dwarfs in the Pleiades Cluster. II. J, H and K photometry

We have obtained near-IR observations of some of the faintest objects so far known towards the Pleiades cluster, with the aim of investigating the sequence that connects cluster very low-mass (VLM) stars with substellar objects. We find that IR data combined with optical magnitudes are a useful tool to discriminate cluster members from foreground/background late-type field stars contaminating optical surveys. The bottom of the Pleiades sequence is clearly defined by the faint HHJ objects as the VLM stars approaching the substellar limit (SL), by the transition object PPl15, which will barely ignite its H content, and by the two brown dwarfs (BDs) Calar3 and Teide1. Binarity amongst cluster members could account for the large dispersion observed in the faint end of the IR colour- magnitude diagrams. Two objects in our sample, namely HHJ6 and PPl15, are overluminous compared to other members, suggesting probable binarity. We have reproduced the photometric measurements of both of them by combining the magnitudes of cluster VLM stars and BDs and using the most recent theoretical evolutionary tracks. The likely masses of the components are slightly above the SL for HHJ6, while they are 0.080 and 0.045+-0.010 Msun for PPl15. These masses are consistent with the constraints imposed by the published Li observations of these Pleiads. We find a single object IR sequence in the Pleiades connecting VLM stars and BDs. We propose that the SL (~0.075 Msun) in the Pleiades (~120 Myr) takes place at absolute magnitudes M(I,J,H,K)=12.4, 10.1, 9.4, 9.0 (~M7). Cluster members fainter by 0.2mag in I and by 0.1mag in K should be proper BDs. The star-BD frontier in the Hyades would be located at M(I,J,H,K)=15.0, 11.6, 10.8, 10.4 (~M9). For ages older than 1000 Myr we estimate that BDs are fainter than M(K)=10.9 (later than M9.5).

astro-ph↗

arXiv1996

Search for young low-mass stars in a ROSAT selected sample south of the Taurus-Auriga molecular clouds

We present results of intermediate resolution spectroscopy of 131 optical counterparts to 115 ROSAT All-Sky Survey X-ray sources south of the Taurus-Auriga dark cloud complex. These objects have been selected as candidate young stars from a total of 1084 ROSAT sources in a about 300 square degree area. We identify 30 objects as low-mass PMS stars on the basis of the Li 6708 doublet in their spectrum, a signature of their young age. All these stars have a spectral type later than F7 and show spectral characteristics typical of weak-line and post-T Tauri stars. The presence of young objects several parsecs away from the regions of ongoing star formation is discussed in the light of the current models of T Tauri dispersal.

astro-ph↗