Richard P. Boyle

PERIODIC BURSTS OF COHERENT RADIO EMISSION FROM AN ULTRACOOL DWARF

We report the detection of periodic (p = 1.96 hr) bursts of extremely bright, 100% circularly polarized, coherent radio emission from the M9 dwarf TVLM 513-46546. Simultaneous photometric monitoring observations have established this periodicity to be the rotation period of the dwarf. These bursts, which were not present in previous observations of this target, confirm that ultracool dwarfs can generate persistent levels of broadband, coherent radio emission, associated with the presence of kG magnetic fields in a large-scale, stable configuration. Compact sources located at the magnetic polar regions produce highly beamed emission generated by the electron cyclotron maser instability, the same mechanism known to generate planetary coherent radio emission in our solar system. The narrow beams of radiation pass our line of sight as the dwarf rotates, producing the associated periodic bursts. The resulting radio light curves are analogous to the periodic light curves associated with pulsar radio emission highlighting TVLM 513-46546 as the prototype of a new class of transient radio source.

Rotational Modulation of M/L Dwarfs Due to Magnetic Spots

We find periodic I-band variability in two ultracool dwarfs, TVLM 513-46546 and 2MASS J00361617+1821104, on either side of the M/L dwarf boundary. Both of these targets are short-period radio transients, with the detected I-band periods matching those found at radio wavelengths (P = 1.96 hr for TVLM 513-46546 and P = 3 hr for 2MASS J00361617+1821104). We attribute the detected I-band periodicities to the periods of rotation of the dwarfs, supported by radius estimates and measured v sin i values for the objects. Based on the detected period of rotation of TVLM 513-46546 (M9) in the I band, along with confirmation of strong magnetic fields from recent radio observations, we argue for magnetically induced spots as the cause of this periodic variability. The I-band rotational modulation of the L3.5 dwarf 2MASS J00361617+1821104 appeared to vary in amplitude with time. We conclude that the most likely cause of the I-band variability for this object is magnetic spots, possibly coupled with time-evolving features such as dust clouds.

Evidence of Halo Microlensing in M31

We have completed an intensive monitoring program of two fields on either side of the center of M31 and report here on the results concerning microlensing of stars in M31. These results stem from a 3 yr study ( the Vatican Advanced Technology Telescope [VATT]/Columbia survey) of microlensing and variability of M31 stars, emphasizing microlensing events of 3 day to 2 month timescales and likely due to masses in M31. These observations were conducted intensively from 1997 to 1999, with baselines 1995 to present, at the VATT and the 1.3 m telescope at MDM Observatory, with additional data from the Isaac Newton Telescope, including about 200 epochs total. The two fields monitored cover 560 arcmin(2) total, positioned along the minor axis on either side of M31. Candidate microlensing events are subject to a number of tests discussed here with the purpose of distinguishing microlensing from variable star activity. A total of four probable microlensing events, when compared with carefully computed event rate and efficiency models, indicate a marginally significant microlensing activity above that expected for the stars alone in M31 ( and the Galaxy) acting as lenses. A maximum likelihood analysis of the distribution of events in timescale and across the face of M31 indicates a microlensing dark matter halo fraction consistent with that seen by MACHO in our Galaxy toward the Large Magellanic Cloud. Specifically, for a nearly singular isothermal sphere model, we find a microlensing halo mass fraction fb = 0.29(-0.13)(+0.30) of the total dark matter and a poorly constrained lensing component mass (0.02 - 1.5 M-.; 1 sigma limits). This study serves as the prototype for a larger study approaching completion; between the two there is significant evidence for an asymmetry in the distribution of microlensing events across the face of M31 and therefore a large population of halo microlensing dark matter objects.

PERIODIC OPTICAL VARIABILITY OF RADIO-DETECTED ULTRACOOL DWARFS

A fraction of very low mass stars and brown dwarfs are known to be radio active, in some cases producing periodic pulses. Extensive studies of two such objects have also revealed optical periodic variability, and the nature of this variability remains unclear. Here, we report on multi-epoch optical photometric monitoring of six radio-detected dwarfs, spanning the ~M8-L3.5 spectral range, conducted to investigate the ubiquity of periodic optical variability in radio-detected ultracool dwarfs. This survey is the most sensitive ground-based study carried out to date in search of periodic optical variability from late-type dwarfs, where we obtained 250 hr of monitoring, delivering photometric precision as low as ~0.15%. Five of the six targets exhibit clear periodicity, in all cases likely associated with the rotation period of the dwarf, with a marginal detection found for the sixth. Our data points to a likely association between radio and optical periodic variability in late-M/early-L dwarfs, although the underlying physical cause of this correlation remains unclear. In one case, we have multiple epochs of monitoring of the archetype of pulsing radio dwarfs, the M9 TVLM 513–46546, spanning a period of 5 yr, which is sufficiently stable in phase to allow us to establish a period of 1.95958 ± 0.00005 hr. This phase stability may be associated with a large-scale stable magnetic field, further strengthening the correlation between radio activity and periodic optical variability. Finally, we find a tentative spin-orbit alignment of one component of the very low mass binary, LP 349–25.

Spin-orbit alignment in the very low mass binary regime The L dwarf tight binary 2MASSW J0746425+200032AB

Studies of solar-type binaries have found coplanarity between the equatorial and orbital planes of systems with <40 AU separation. By comparison, the alignment of the equatorial and orbital axes in the substellar regime, and the associated implications for formation theory, are relatively poorly constrained. Here we present the discovery of the rotation period of 3.32 ±u2009 0.15 h from 2MASS J0746+20A – the primary component of a tight (2.7 AU) ultracool dwarf binary system (L0+L1.5). The newly discovered period, together with the established period via radio observations of the other component, and the well constrained orbital parameters and rotational velocity measurements, allow us to infer alignment of the equatorial planes of both components with the orbital plane of the system to within 10 degrees. This result suggests that solar-type binary formation mechanisms may extend down into the brown dwarf mass range, and we consider a number of formation theories that may be applicable in this case. This is the first such observational result in the very low mass binary regime. In addition, the detected period of 3.32 ± 0.15 h implies that the reported radio period of 2.07 ± 0.002 h is associated with the secondary star, not the primary, as was previously claimed. This in turn refutes the claimed radius of 0.78 ± 0.1 R_J for 2MASS J0746+20A, which we demonstrate to be 0.99 ± 0.03 R_J.

Progress at the Vatican Advanced Technology Telescope

The Vatican Advanced Technology Telescope incorporates a fast (f/1.0) borosilicate honeycomb primary mirror and an f/0.9 secondary in an aplanatic Gregorian optical configuration. We provide a brief technical and performance overview by describing the optical layout, the primary and secondary mirror systems, and the telescope drive and control system. Results from a high resolution wavefront sensor and a current wide-field image taken at the f/9 focus demonstrates the overall fine performance of the telescope.

Radial velocities of K–M dwarfs and local stellar kinematics

Aims. The goal of this paper is to present complete radial-velocity data for the spectroscopically selected McCormick sample of nearby K–M dwarfs and, based on these and supplementary data, to determine the space-velocity distributions of late-type stars in the solar neighborhood. Methods. We analyzed nearly 3300 measurements of radial velocities for 1049 K–M dwarfs, that we obtained during the past decade with a CORAVEL-type instrument, with a primary emphasis on detecting and eliminating from kinematic calculations the spectroscopic binaries and binary candidates. Combining radial-velocity data with Hipparcos/Tycho-2 astrometry we calculated the space-velocity components and parameters of the galactic orbits in a three-component model potential for the stars in the sample, that we use for kinematical analysis and for the identification of possible candidate members of nearby stellar kinematic groups. Results. We present the catalog of our observations of radial velocities for 959 stars which are not suspected of velocity variability, along with the catalog of U,V,W velocities and Galactic orbital parameters for a total of 1088 K–M stars which are used in the present kinematic analysis. Of these, 146 stars were identified as possible candidate members of the known nearby kinematic groups and suspected subgroups. The distributions of space-velocity components, orbital eccentricities, and maximum distances from the Galactic plane are consistent with the presence of young, intermediate-age and old populations of the thin disk and a small fraction (~3%) of stars with the thick disk kinematics. The kinematic structure gives evidence that the bulk of K–M type stars in the immediate solar vicinity represents a dynamically relaxed stellar population. The star MCC 869 is found to be on a retrograde Galactic orbit ( V = −262 kmu2009s -1 ) of low inclination (4°) and can be a member of stellar stream of some dissolved structure. The Sun’s velocity with respect to the Local Standard of Rest, derived from the distributions of space-velocity components, is ( U ⊙ , V ⊙ , W ⊙ ) = (9.0 ± 1.4,13.1 ± 0.6,7.2 ± 0.8) kmu2009s -1 . The radial solar motion derived via the Stromberg’s relation, V ⊙ = 14.2 ± 0.8 kmu2009s -1 , agrees within the errors with the value obtained directly from the V distribution of stars on nearly circular orbits.

Variable stars from two surveys of microlensing in M31

Recently we completed the VATT/Columbia survey monitoring a portion of M31 for microlensing and stellar variability, and report here on the microlensing result. Also, we have nearly completed a much larger survey (MEGA) covering most of the stars in M31, for which we have started cataloging many tens of thousands of Cepheids, Miras, eclipsing variables and other kinds of variable stars. The microlensing results from these two surveys hint that a significant fraction of the dark matter in M31 is composed of stellar-mass objects in the halo. 1. Surveys: description, analysis and results The composition of dark matter in disk galaxies remains a mystery. The MACHO survey revealed a microlensing rate unanticipated in the context of the visible stellar population alone, that may compose a significant part of the dark matter halo (Alcock et al. 2000), consistent with only upper limits from EROS (Afonso et al. 2003). A decade ago we proposed that M31 offers a favorable alternative for probing the halo dark matter in disk galaxies, using microlensing of stars in M31 itself for lenses primarily in M31. Such a signal could be easily distinguished in terms of an asymmetry in microlensing rate across the face of M31 (Crotts 1992). This required developing techniques for the application of image subtraction to a time series of images (Tomaney & Crotts 1996), which led to the first candidate microlensing events in M31 (Crotts & Tomaney 1996). Several years of such observations were required both to amass sufficient lensing events for a statistically meaningful sample and to cull out variable stars by extending the baseline. M31 microlensing surveys have also been conducted by the AGAPE groups (Calchi Novati et al. 2002), and the survey reported here has been extended (de Jong et al. 2003). Together our two surveys hint that much of the dark halo might be composed of stellar-mass lensing objects. The VATT/Columbia survey consisted of over 200 epochs of observation on three telescopes (the 1.8-m Vatican Advanced Technology Telescope (VATT) on Mt. Graham, Arizona; the MDM 1.3-m McGraw-Hill telescope on Kitt Peak, Arizona, with additional data from the Isaac Newton Telescope (INT) on La Palma, Canary Islands). Data were taken in R and I bands in two fields on either side of M31 along its minor axis placed so as to maximize the effects of