Publications (32)32.17 Total impact
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Article: Herschel PACS and SPIRE Observations of TWA brown dwarf discs
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ABSTRACT: We present Herschel SPIRE observations for the TW Hydrae association (TWA) brown dwarf discs SSSPM J1102-3431 (SS1102) and 2MASSW J1207334-393254 (2M1207). Both discs are undetected in the SPIRE 200-500mu bands. We have also analyzed the archival PACS data and find no detection for either source in the 160mu band. Based on radiative transfer modeling, we estimate an upper limit to the disc mass for both sources of 0.1 M_Jup. The lack of detection in the SPIRE bands could be due to a paucity of millimeter sized dust grains in the 2M1207 and SS1102 discs. We also report a non-detection for the brown dwarf 2MASS J1139511-315921 (2M1139) in the PACS 70 and 160mu bands. We have argued for the presence of a warm debris disc around 2M1139, based on an excess emission observed at 24mu. The mid-infrared colors for 2M1139 are similar to the transition discs in the Taurus and Ophuichus regions. A comparison of the brown dwarf disc masses over a ~1-10 Myr age interval suggests a decline in the disc mass with the age of the system.06/2012; -
Article: Revised analysis of SPIRE observations for 2M1207
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ABSTRACT: We have revised our analysis of the SPIRE observations of 2MASSW J1207334-393254 (2M1207). Recent PACS observations show a bright source located ~25" east of 2M1207. There are issues in terms of the detection/non-detection of the bright source when comparing the Spitzer, WISE, and PACS observations. It is apparently inconsistent, perhaps due to variability or low signal-to-noise of the data. We have looked into the possible misidentification of the target, and have revised the measured SPIRE fluxes and the disc parameters for 2M1207. We have also reviewed which among the various formation mechanisms of this system would still be valid.01/2012; -
Article: Simultaneous Multi-Wavelength Observations of Magnetic Activity in Ultracool Dwarfs. III. X-ray, Radio, and H-alpha Activity Trends in M and L Dwarfs
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ABSTRACT: [Abridged] As part of our on-going investigation into the magnetic field properties of ultracool dwarfs, we present simultaneous radio, X-ray, and H-alpha observations of three M9.5-L2.5 dwarfs (BRI0021-0214, LSR060230.4+391059, and 2MASSJ052338.2-140302). We do not detect X-ray or radio emission from any of the three sources, despite previous detections of radio emission from BRI0021 and 2M0523-14. Steady and variable H-alpha emission are detected from 2M0523-14 and BRI0021, respectively, while no H-alpha emission is detected from LSR0602+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 both X-rays and H-alpha roughly follows L_{X,Halpha}/L_bol ~ 10^[-0.4x(SP-M6)] for SP>M6. 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-alpha observations of ultracool dwarfs, and reveals a clear breakdown of the radio/X-ray correlation beyond spectral type M7, evolving smoothly from L_{\nu,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. Comment: Submitted to ApJ; 19 pages, 10 figures, 5 tables09/2009; -
Article: Brown Dwarf Disks at Ages of 5-10 Myr
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ABSTRACT: We present Spitzer/IRAC and MIPS 24mu archival observations for 28 brown dwarfs in the Upper Scorpius (UppSco) region. We find a disk fraction of 10.7%^(+8.7%)_(-3.3%). One object shows a small excess at 24mu but none at shorter wavelengths, and may be a candidate transition disk. Three objects show emission in the 10mu silicate feature and we present compositional fits for these sources. Flat structures are observed for all disk sources in UppSco. Also presented are the MIPS/70mu observations for the TW Hydrae Association brown dwarf 2MASS J1139511-315921. We discuss the structure and chemistry of brown dwarf disks at ages of ~5-10 Myr, and consider the possible effects of the brown dwarf densities in these clusters on the disk lifetimes. Comment: Accepted in ApJ09/2009; -
Article: L Dwarfs and the Substellar Mass Function
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ABSTRACT: Analysis of initial observations sky surveys has shown that the resulting photometric catalogs, combined with far-red optical data, provide an extremely effective method of finding isolated, very low-temperature objects in the general field. Follow-up observations have already identified more than 25 sources with temperatures cooler than the latest M dwarfs. A comparison with detailed model predictions (Burrows & Sharp 1999) indicates that these L dwarfs have effective temperatures between ≈2000 ± 100 K and 1500 ± 100 K, while the available trigonometric parallax data place their luminosities at between 10-3.5 and 10. Those properties, together with the detection of lithium in one-third of the objects, are consistent with the majority having substellar masses. The mass function cannot be derived directly, since only near-infrared photometry and spectral types are available for most sources, but we can incorporate VLM/brown dwarf models in simulations of the solar neighborhood population and constrain Ψ(M) by comparing the predicted L dwarf surface densities and temperature distributions against observations from the Deep Near-Infrared Survey (DENIS) and 2 Micron All-Sky Survey (2MASS) surveys. The data, although sparse, can be represented by a power-law mass function, Ψ(M) M-α, with 1 < α < 2. Current results favor a value nearer the lower limit. If α = 1.3, then the local space density of 0.075 > M/M☉ > 0.01 brown dwarfs is 0.10 systems pc-3. In that case, brown dwarfs are twice as common as main-sequence stars but contribute no more than ~15% of the total mass of the disk.The Astrophysical Journal 01/2009; 521(2):613. · 6.02 Impact Factor -
Article: Simultaneous Multiwavelength Observations of Magnetic Activity in Ultracool Dwarfs. II. Mixed Trends in VB 10 and LSR 1835+32 and the Possible Role of Rotation
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ABSTRACT: As part of our ongoing investigation of magnetic activity in ultracool dwarfs we present simultaneous radio, X-ray, UV, and optical observations of LSR1835+32 (M8.5), and simultaneous X-ray and UV observations of VB10 (M8), both with a duration of about 9 hr. LSR1835+32 exhibits persistent radio emission, and Hα variability on timescales of 0.5-2 hr. The detected UV flux is consistent with photospheric emission, and no X-ray emission is detected to a deep limit of LX/Lbol 10−5.7. The Hα and radio emission are temporally uncorrelated, and the ratio of radio to X-ray luminosity exceeds the correlation seen in F-M6 stars by >2 × 104. The lack of radio variability during four rotations of LSR1835+32 requires a uniform stellar-scale field of ~10 G, and indicates that the Hα variability is dominated by much smaller scales, <10% of the chromospheric volume. VB10, on the other hand, shows correlated flaring and quiescent X-ray and UV emission, similar to the behavior of early M dwarfs. Delayed and densely sampled optical spectra exhibit a similar range of variability amplitudes and timescales. Along with our previous observations of the M8.5 dwarf TVLM513–46546 we conclude that late M dwarfs exhibit a mix of activity patterns, which points to a transition in the structure and heating of the outer atmosphere by large-scale magnetic fields. We find that rotation may play a role in generating the fields as evidenced by a tentative correlation between radio activity and rotation velocity. The X-ray emission, however, shows evidence for supersaturation at vsin i > 25 km s−1, which could be the result of secondary effects such as inefficient heating or centrifugal stripping of extended coronal loops. These effects may underlie the severe violation of the radio/X-ray correlation in ultracool dwarfs.The Astrophysical Journal 12/2008; 676(2):1307. · 6.02 Impact Factor -
Article: Simultaneous Multiwavelength Observations of Magnetic Activity in Ultracool Dwarfs. I. The Complex Behavior of the M8.5 Dwarf TVLM 513–46546
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ABSTRACT: We present the first simultaneous radio, X-ray, ultraviolet, and optical spectroscopic observations of the M8.5 dwarf TVLM 513–46546, with a duration of 9 hr. These observations are part of a program to study the origin of magnetic activity in ultracool dwarfs, and its impact on chromospheric and coronal emission. Here we detect steady quiescent radio emission superposed with multiple short-duration, highly polarized flares; there is no evidence for periodic bursts previously reported for this object, indicating their transient nature. We also detect soft X-ray emission, with LX/Lbol ≈ 10−5.1, the faintest to date for any object later than M5, and a possible X-ray flare. TVLM 513–46546 continues the trend of severe violation of the radio/X-ray correlation in ultracool dwarfs, by nearly 4 orders of magnitude. From the optical spectroscopy we find that the Balmer line luminosity exceeds the X-ray luminosity by a factor of a few, ruling out chromospheric heating by coronal X-ray emission. More importantly, we detect sinusoidal Hα and Hβ equivalent width light curves with a period of 2 hr, matching the rotation period of TVLM 513–46546. This behavior points to a corotating chromospheric hot spot or an extended magnetic structure, with a covering fraction of about 50%. This feature may be transitory based on the apparent decline in light-curve peak during the four observed maxima. From the radio data we infer a large-scale and steady magnetic field of ~102 G. A large-scale field is also required by the sinusoidal Balmer line emission. The radio flares, on the other hand, are produced in a component of the field with a strength of ~3 kG and a likely multipolar configuration. The overall lack of correlation between the various activity indicators suggests that the short-duration radio flares do not have a strong influence on the chromosphere and corona, and that the chromospheric emission is not the result of coronal heating.The Astrophysical Journal 12/2008; 673(2):1080. · 6.02 Impact Factor -
Article: The Magnetic Properties of an L Dwarf Derived from Simultaneous Radio, X-Ray, and Hα Observations
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ABSTRACT: 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.The Astrophysical Journal 12/2008; 627(2):960. · 6.02 Impact Factor -
Article: Periodic Radio and H-alpha Emission from the L Dwarf Binary 2MASSW J0746425+200032: Exploring the Magnetic Field Topology and Radius of an L Dwarf
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ABSTRACT: [Abridged] We present an 8.5-hour simultaneous radio, X-ray, UV, and optical observation of the L dwarf binary 2MASSW J0746+20. 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 non-variable component is also detected, likely due to emission from a uniform large-scale field. The H-alpha 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 mis-aligned 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 (i) the field is dominated by a quadrupole configuration, which can naturally explain the 1/4 phase lag; or (ii) the H-alpha 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 (vsini=27 km/s), and the binary orbital inclination (i=142 deg), to derive a radius for the primary star of 0.078+/-0.010 R_sun. This is the first measurement of the radius of an L dwarf, and along with a mass of 0.085+/-0.010 M_sun it provides a constraint on the mass-radius relation below 0.1 M_sun. We find that the radius is about 30% smaller than expected from theoretical models, even for an age of a few Gyr.08/2008; -
Article: New Brown Dwarf Disks in the TW Hydrae Association
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ABSTRACT: In our analysis of {\it Spitzer}/IRS archival data on the stellar and sub-stellar members of the TW Hydrae Association (TWA), we have discovered two new brown dwarf disks: a flat optically thick disk around SSSPM J1102-3431 (SSSPM 1102), and a transition disk around 2MASS J1139511-315921 (2M1139). The disk structure for SSSPM 1102 is found to be very similar to the known brown dwarf disk 2MASSW J1207334-393254 (2M1207), with excess emission observed at wavelengths as short as 5 $\micron$. 2M1139 shows no excess emission shortward of $\sim$20 $\micron$, but flares up at longer wavelengths, and is the first transition disk detected among the sub-stellar members of TWA. We also report on the {\it Spitzer}/70 $\micron$ observations, and the presence of a weak {\it absorption} 10 $\micron$ silicate feature for 2M1207. The absorption can be attributed to a close to edge-on disk at a 75$\degr$ inclination. The 10 $\micron$ spectrum for 2M1207 shows crystalline forsterite features, with a peak in absorption near 11.3 $\micron$. No silicate absorption/emission is observed towards SSSPM 1102. While only 6 out of 25 stellar members show excess emission at these mid-infrared wavelengths, {\it all} of the TWA brown dwarfs that have been observed so far with {\it Spitzer} show signs of disks around them, resulting in a disk fraction of at least 60%. This is a considerable fraction at a relatively older age of $\sim$10 Myr. A comparison with younger clusters indicates that by the age of the TWA ($\sim$10 Myr), the disk fraction for brown dwarfs has not decreased, whereas it drops by a factor of $\sim$2 for the higher mass stars. This suggests longer disk decay time scales for brown dwarfs compared to higher mass stars. Comment: 23 pages, 6 figures. Accepted in ApJ02/2008; -
Article: Four Nearby L Dwarfs
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ABSTRACT: We present spectroscopic, photometric and astrometric observations of four bright L dwarfs identified in the course of the 2MASS near-infrared survey. Our spectroscopic data extend to wavelengths shortward of 5000 Å in the L0 dwarf 2MASS J0746+2000 and the L4 dwarf 2MASS J0036+1840, allowing the identification of absorption bands due to MgH and CaOH. The atomic resonance lines Ca I λ4227 and Na I λλ5890/5896 are extremely strong, with the latter having an equivalent width of 240 Å in the L4 dwarf. By spectral type L5, the D lines extend over ~1000 Å and absorb a substantial fraction of the flux emitted in the V band, with a corresponding effect on the (V-I) broadband color. The K I resonance doublet at 7665/7699 Å increases in equivalent width from spectral type M3 to M7, but decreases in strength from M7 to L0 before broadening substantially at later types. These variations are likely driven by dust formation in these cool atmospheres.The Astronomical Journal 12/2007; 119(1):369. · 4.03 Impact Factor -
Article: High-Resolution Spectroscopy of Ultracool M Dwarfs
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ABSTRACT: We present high-resolution echelle spectroscopy of 39 dwarfs with spectral types between M6.5 and L0.5. With one exception, those dwarfs were selected from the Two Micron All Sky Survey database using photometric criteria, (J-KS) ≥ 1.1 and KS ≤ 12.0, and therefore should provide a sample free of the kinematic biases that can affect proper-motion–selected samples. Two of the stars, 2MASSI J0253202+271333 and 2MASSW J0952219-192431, are double-lined spectroscopic binaries. We have used our observations to search for Li I 6708 Å absorption, characteristic of substellar mass; estimate the level of chromospheric activity through measurement of Hα emission fluxes; measure rotational velocities via line broadening; and determine radial velocities and Galactic space motions. Two dwarfs have strong lithium absorption, the previously known brown dwarf LP 944-20 and 2MASSI J0335020+234235, which we identify as a probable 0.06 M brown dwarf with an age of ~1 Gyr. We have investigated the prospect of using the observed frequency of lithium absorption among ultracool M dwarfs (M7 to M9.5) as a probe of the initial mass function, comparing the observed frequency against predictions based on recent theoretical models of low-mass dwarfs and an assumed star formation history. Our results show that the conclusions drawn are vulnerable both to systematic differences between the available models and to incomplete local sampling of the most recent star formation events (ages less than 108 yr). The latter consideration stems from the mass-dependent rate of evolution of brown dwarfs. Even given those caveats, however, the available observations are difficult to reconcile with Salpeter-like power-law mass functions (α ≥ 2) for masses below 0.1 M. A comparison between the rotational velocities and Hα fluxes shows no evidence for significant correlation. The mean activity level of the ultracool dwarfs lies almost a factor of 10 below that of early- and mid-type M dwarfs. The relative number of dwarfs with v sin i < 20 km s-1 with respect to greater than 20 km s-1 is independent of spectral type. Finally, velocity dispersions derived for our photometrically selected sample of ultracool dwarfs are significantly lower than those measured for nearby M dwarfs but show remarkable similarity to results for earlier type emission-line (dMe) dwarfs. The latter are generally assigned ages of less than ~3 Gyr.The Astronomical Journal 12/2007; 124(1):519. · 4.03 Impact Factor -
Article: Simultaneous Multi-Wavelength Observations of Magnetic Activity in Ultracool Dwarfs. II. Mixed Trends in VB10 and LSR1835+32 and the Possible Role of Rotation
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ABSTRACT: [Abridged] As part of our on-going investigation of magnetic activity in ultracool dwarfs we present simultaneous radio, X-ray, UV, and optical observations of LSR1835+32 (M8.5), and simultaneous X-ray and UV observations of VB10 (M8), both with a duration of about 9 hours. LSR1835+32 exhibits persistent radio emission and H-alpha variability on timescales of ~0.5-2 hr. The detected UV flux is consistent with photospheric emission, and no X-ray emission is detected to a deep limit of L_X/L_bol<10^-5.7. The H-alpha and radio emission are temporally uncorrelated, and the ratio of radio to X-ray luminosity exceeds the correlation seen in F-M6 stars by >2x10^4. Similarly, L_Halpha/L_X>10 is at least 30 times larger than in early M dwarfs, and eliminates coronal emission as the source of chromospheric heating. The lack of radio variability during four rotations of LSR1835+32 requires a uniform stellar-scale field of ~10 G, and indicates that the H-alpha variability is dominated by much smaller scales, <10% of the chromospheric volume. VB10, on the other hand, shows correlated flaring and quiescent X-ray and UV emission, similar to the behavior of early M dwarfs. Delayed and densely-sampled optical spectra exhibit a similar range of variability amplitudes and timescales to those seen in the X-rays and UV, with L_Halpha/L_X~1. Along with our previous observations of the M8.5 dwarf TVLM513-46546 we conclude that late M dwarfs exhibit a mix of activity patterns, which points to a transition in the structure and heating of the outer atmosphere by large-scale magnetic fields. We find that rotation may play a role in generating the fields as evidenced by a tentative correlation between radio activity and rotation velocity. The X-ray emission, however, shows evidence for super-saturation at vsini>25 km/s. Comment: Submitted to ApJ10/2007; -
Article: A Chandra X-ray detection of the L dwarf binary Kelu-1: Simultaneous Chandra and Very Large Array observations
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ABSTRACT: Magnetic activity in ultracool dwarfs, as measured in X-rays and H$\alpha$, shows a steep decline after spectral type M7-M8. So far, no L dwarf has been detected in X-rays. In contrast, L dwarfs may have higher radio activity than M dwarfs. We observe L and T dwarfs simultaneously in X-rays and radio to determine their level of magnetic activity in the context of the general decline of magnetic activity with cooler effective temperatures. The field L dwarf binary Kelu-1 was observed simultaneously with Chandra and the Very Large Array. Kelu-1AB was detected in X-rays with $L_{\rm X} = 2.9_{-1.3}^{+1.8} \times 10^{25}$ erg/s, while it remained undetected in the radio down to a $3 \sigma$ limit of $L_{\rm R} \leq 1.4 \times 10^{13}$ erg/s/Hz. We argue that, whereas the X-ray and H$\alpha$ emissions decline in ultracool dwarfs with decreasing effective temperature, the radio luminosity stays (more or less) constant across M and early-L dwarfs. The radio surface flux or the luminosity may better trace magnetic activity in ultracool dwarfs than the ratio of the luminosity to the bolometric luminosity. Deeper radio observations (and at short frequencies) are required to determine if and when the cut-off in radio activity occurs in L and T dwarfs, and what kind of emission mechanism takes place in ultracool dwarfs. Comment: Accepted for publication as a Letter in Astronomy & Astrophysics07/2007; -
Chapter: 2MASS Brown Dwarfs and a First Estimate of the Substellar Mass Function
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ABSTRACT: More than 70 L dwarfs have now been found in followup work on a small portion of the sky surveyed by the Two Micron All Sky Survey. It is estimated that the L dwarfs span a temperature range of 1400 to 2000K. Several have trigonometric parallax determinations, and luminosities between 10-3.5 and 10-4.3 L \odot_\odot . These properties, together with the detection of lithium in one third of the objects, are consistent with the majority having substellar masses. Evolutionary models are utilized to derive estimates of the space density and mass function of substellar objects, predicting a majority that are not detectable with 2MASS. In a given volume the brown dwarfs may outnumber the stars, though contributing only modestly to the local disk mass.11/2006: pages 505-510; -
Article: A Deep Look at the T-Type Brown Dwarf Binary $\epsilon$ Indi Bab with Chandra and ATCA
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ABSTRACT: We present deep observations of the nearby T-type brown dwarf binary $\epsilon$ Indi Bab in radio with the Australia Telescope Compact Array and in X-rays with the Chandra X-ray Observatory. Despite long integration times, the binary (composed of T1 and T6 dwarfs) was not detected in either wavelength regime. We reached $3\sigma$ upper limits of $1.23 \times 10^{12}$ and $1.74 \times 10^{12}$ erg/s/Hz for the radio luminosity at 4.8 GHz and 8.64 GHz, respectively; in the X-rays, the upper limit in the $0.1-10$ keV band was $3.16 \times 10^{23}$ erg/s. We discuss the above results in the framework of magnetic activity in ultracool, low-mass dwarfs. Comment: Accepted for publication in the Astrophysical Journal Letters04/2005; -
Article: New M and L Dwarfs Confirmed with CorMASS
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ABSTRACT: We present new M and L-dwarfs confirmed through follow-up of 2MASS color-selected objects with the CorMASS near-infrared spectrograph (R ˜300) on the Palomar 60-inch telescope as part of a continuing follow-up survey. Most of the objects are bright (Ks < 13).Proceedings of the International Astronomical Union 05/2003; 211:197. -
Article: Hubble Space Telescope Observations of Binary Very-Low-Mass Stars and Brown Dwarfs
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ABSTRACT: We present analysis of Hubble Space Telescope images of 82 nearby field late-M and L dwarfs. We resolve 13 of these systems into double M/L dwarf systems and identify an additional possible binary. Combined with previous observations of 20 L dwarfs, we derive an observed binary fraction for ultracool dwarfs of 17+4-3%, where the statistics included systems with separations in the range 1.6-16 A.U. We argue that accounting for biases and incompleteness leads to an estimated binary fraction 15+-5% in the range 1.6-16 A.U. No systems wider than 16 A.U. are seen, implying that the wide companion frequency is less than 1.7%; the distribution of orbital separation is peaked at ~2-4 A.U. and differs greatly from the G dwarf binary distribution. Indirect evidence suggests that the binary fraction is ~5+-3% for separations less than 1.6 A.U. We find no evidence for differences in the binary fraction between stellar late-M and L dwarfs and substellar L dwarfs. We note, however, that the widest (>10 A.U.) systems in our sample are all of earlier (M8-L0) spectral type; a larger sample is needed determine if this is a real effect. One system with a spectral type of L7 has a secondary that is fainter in the HST F814W filter but brighter in F1042M; we argue that this secondary is an early-T dwarf. Comment: To appear in June 2003 Astronomical Journal02/2003; -
Article: Astrometry and Photometry for Cool Dwarfs and Brown Dwarfs
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ABSTRACT: Trigonometric parallax determinations are presented for 28 late type dwarfs and brown dwarfs, including eight M dwarfs with spectral types between M7 and M9.5, 17 L dwarfs with spectral types between L0 and L8, and three T dwarfs. Broadband photometry at CCD wavelengths (VRIz) and/or near-IR wavelengths (JHK) are presented for these objects and for 24 additional late-type dwarfs. Supplemented with astrometry and photometry from the literature, including ten L and two T dwarfs with parallaxes established by association with bright, usually HIPPARCOS primaries, this material forms the basis for studying various color-color and color-absolute magnitude relations. The I-J color is a good predictor of absolute magnitude for late-M and L dwarfs. M_J becomes monotonically fainter with I-J color and with spectral type through late-L dwarfs, then brightens for early-T dwarfs. The combination of zJK colors alone can be used to classify late-M, early-L, and T dwarfs accurately, and to predict their absolute magnitudes, but is less effective at untangling the scatter among mid- and late-L dwarfs. The mean tangential velocity of these objects is found to be slightly less than that for dM stars in the solar neighborhood, consistent with a sample with a mean age of several Gyr. Using colors to estimate bolometric corrections, and models to estimate stellar radii, effective temperatures are derived. The latest L dwarfs are found to have T_eff ~ 1360 K. Comment: 48 pages, including 7 figures and 6 tables. Accepted for AJ05/2002; -
Article: The Palomar/MSU Nearby Star Spectroscopic Survey III: Chromospheric Activity, M-dwarf Ages and the Local Star Formation History
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ABSTRACT: We present high-resolution echelle spectroscopy of 676 nearby M dwarfs. Our measurements include radial velocities, equivalent widths of important chromospheric emission lines, and rotational velocities for rapidly rotating stars. We identify several distinct groups by their H alpha properties, and investigate variations in chromospheric activity amongst early (M0-M2.5) and mid (M3-M6) dwarfs. Using a volume-limited sample together with a relationship between age and chromospheric activity, we show that the rate of star formation in the immediate Solar Neighbourhood has been relatively constant over the last 4 Gyr. In particular our results are inconsistent with recent large bursts of star formation. We use the correlation between H alpha activity and age as a function of colour to set constraints on the properties of L and T dwarf secondary components in binary systems. We also identify a number of interesting stars, including rapid rotators, radial velocity variables, and spectroscopic binaries. Comment: accepted for A.J., 80 pages with large tables03/2002;
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2006–2009
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University of Massachusetts Amherst
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2007–2008
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University of Delaware
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Newark, DE, USA
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