Eduardo L. Martin

Spectroscopy of a brown dwarf candidate in the Alpha Persei open cluster

We report intermediate-resolution spectroscopy in the range 625-775 nm of a faint (R∼19) very late type object, discovered in two CCD images taken near the center of the α Per cluster. The spectrum shows strong absorption molecular bands, Hα emission, and KI absorption typical of late-M dwarfs; Hα varies conspicously (W λ ∼2.4-0.8 nm) is a short time scale. The effective temperature and luminosity derived from the spectrum and R,I band photometry are consistent with those expected for an α Per (age∼50 Myr) 0.07 M ⊙ substellar object

Spectroscopy of new substellar candidates in the pleiades: towards a spectral sequence for young brown dwarfs

We present optical and near-IR spectroscopy (600--1000 nm) of 8 faint (I

Simultaneous Multi-Wavelength Observations of Magnetic Activity in Ultracool Dwarfs. III. X-ray, Radio, and Hα Activity Trends in M and L dwarfs

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PERIODIC RADIO AND Hα EMISSION FROM THE L DWARF BINARY 2MASSW J0746425+200032: EXPLORING THE MAGNETIC FIELD TOPOLOGY AND RADIUS OF AN L DWARF

18) very red (R--I

Spectroscopic Rotational Velocities of Brown Dwarfs

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The Magnetic Properties of an L Dwarf Derived from Simultaneous Radio, X-Ray, and Hα Observations

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.

A spectroscopic test for substellar objects

As part of our on-going investigation into the magnetic field properties of ultracool dwarfs, we present simultaneous radio, X-ray, and H? observations of three M9.5-L2.5 dwarfs (BRI?0021-0214, LSR?060230.4+391059, and 2MASS?J052338.2?140302). We do not detect X-ray or radio emission from any of the three sources, despite previous detections of radio emission from BRI?0021 and 2M0523?14. Steady and variable H? emission are detected from 2M0523?14 and BRI?0021, respectively, while no H? emission is detected from LSR?0602+39. Overall, our survey of nine M8-L5 dwarfs doubles the number of ultracool dwarfs observed in X-rays, and triples the number of L dwarfs, providing in addition the deepest limits to date, log(L X/L bol) ?5. With this larger sample we find the first clear evidence for a substantial reduction in X-ray activity, by about two orders of magnitude, from mid-M to mid-L dwarfs. We find that the decline in H? roughly follows L H?/L bol 10?0.4?(SP-6) for SP ? 6, where SP = 0 for spectral type M0. In the radio band, however, the luminosity remains relatively unchanged from M0 to L4, leading to a substantial increase in L rad/L bol. Our survey also provides the first comprehensive set of simultaneous radio/X-ray/H? observations of ultracool dwarfs, and reveals a clear breakdown of the radio/X-ray correlation beyond spectral type M7, evolving smoothly from L ?,rad/L X 10?15.5 to ~10?11.5 Hz?1 over the narrow spectral-type range M7-M9. This breakdown reflects the substantial reduction in X-ray activity beyond M7, but its physical origin remains unclear since, as evidenced by the uniform radio emission, there is no drop in the field dissipation and particle acceleration efficiency. Based on the results of our survey, we conclude that a further investigation of magnetic activity in ultracool dwarfs will benefit from a two-pronged approach: multi-rotation observations of nearby known active sources and a snapshot survey of a large sample within ~50 pc to uncover rare flaring objects.

Simultaneous Multiwavelength Observations of Magnetic Activity in Ultracool Dwarfs. I. The Complex Behavior of the M8.5 Dwarf TVLM 513–46546

We present an 8.5 hr simultaneous radio, X-ray, UV, and optical observation of the L dwarf binary 2MASSW J0746425+200032. We detect strong radio emission, dominated by short-duration periodic pulses at 4.86 GHz with P = 124.32 ± 0.11 min. The stability of the pulse profiles and arrival times demonstrates that they are due to the rotational modulation of a B ≈ 1.7 kG magnetic field. A quiescent nonvariable component is also detected, likely due to emission from a uniform large-scale field. The Hα emission exhibits identical periodicity, but unlike the radio pulses it varies sinusoidally and is offset by exactly 1/4 of a phase. The sinusoidal variations require chromospheric emission from a large-scale field structure, with the radio pulses likely emanating from the magnetic poles. While both light curves can be explained by a rotating misaligned magnetic field, the 1/4 phase lag rules out a symmetric dipole topology since it would result in a phase lag of 1/2 (poloidal field) or zero (toroidal field). We therefore conclude that either (1) the field is dominated by a quadrupole configuration, which can naturally explain the 1/4 phase lag; or (2) the Hα and/or radio emission regions are not trivially aligned with the field. Regardless of the field topology, we use the measured period along with the known rotation velocity (v sin i ≈ 27 km s–1), and the binary orbital inclination (i ≈ 142°), to derive a radius for the primary star of 0.078 ± 0.010 R ☉. This is the first measurement of the radius of an L dwarf, and along with a mass of 0.085 ± 0.010 M ☉ it provides a constraint on the mass-radius relation below 0.1 M ☉. We find that the radius is about 30% smaller than expected from theoretical models, even for an age of a few Gyr. The origin of this discrepancy is either a breakdown of the models at the bottom of the main sequence, or a significant misalignment between the rotational and orbital axes.

The physical properties of four ~600 K T dwarfs

We have obtained projected rotation velocities (vrot sin i) of a sample of 19 ultracool dwarfs with spectral types in the interval M6.5-T8 using high-resolution, near-infrared spectra obtained with NIRSPEC and the Keck II telescope. Among our targets there are two young brown dwarfs, two likely field stars, and 15 likely brown dwarfs (30-72 MJup) in the solar neighborhood. Our results indicate that the T-type dwarfs are fast rotators in marked contrast to M-type stars. We have derived vrot sin i velocities between ≤15 and 40 km s-1 for them and have found no clear evidence for T dwarfs rotating strongly faster than L dwarfs. However, there is a hint for an increasing lower envelope on moving from mid-M to L spectral types in the vrot sin i-spectral-type diagram that was previously reported in the literature; our vrot sin i results extend it to even cooler types. Assuming that field brown dwarfs have a size of 0.08-0.1 R☉, we can place an upper limit of 12.5 hr on the equatorial rotation period of T-type brown dwarfs. In addition, we have compared our vrot sin i measurements to spectroscopic rotational velocities of very young brown dwarfs of similar mass available in the literature. The comparison, although model dependent, suggests that brown dwarfs lose some angular momentum during their contraction; however, their spin-down time seems to be significantly longer than that of solar-type to early M stars.

Candidate free-floating super-Jupiters in the young

We present the first simultaneous, multiwavelength observations of an L dwarf, the L3.5 candidate brown dwarf 2MASS J00361617+1821104, conducted with the Very Large Array, the Chandra X-Ray Observatory, and the Kitt Peak 4 m telescope. We detect strongly variable and periodic radio emission (P = 3 hr) with a fraction of about 60% circular polarization. No X-ray emission is detected to a limit of LX/Lbol 2 × 10-5, several hundred times below the saturation level observed in early M dwarfs. Similarly, we do not detect Hα emission to a limit of LHα/Lbol 2 × 10-7, the deepest for any L dwarf observed to date. The ratio of radio to X-ray luminosity is at least 4 orders of magnitude in excess of that observed in a wide range of active stars (including M dwarfs), providing the first direct confirmation that late-M and L dwarfs violate the radio/X-ray correlation. The radio emission is due to gyrosynchrotron radiation in a large-scale magnetic field of about 175 G, which is maintained on timescales longer than 3 yr. The detected 3 hr period may be due to (1) the orbital motion of a companion at a separation of about 5 stellar radii, similar to the configuration of RS CVn systems, (2) an equatorial rotation velocity of about 37 km s-1 and an anchored, long-lived magnetic field, or (3) periodic release of magnetic stresses in the form of weak flares. In the case of orbital motion, the magnetic activity may be induced by the companion, possibly explaining the unusual pattern of activity and the long-lived signal. We conclude that fully convective stars can maintain a large-scale and stable magnetic field, but the lack of X-ray and Hα emission indicates that the atmospheric conditions are markedly different than in early-type stars and even M dwarfs. Similar observations are therefore invaluable for probing both the internal and external structure of low-mass stars and substellar objects, and for providing constraints on dynamo models.