1,721,019 research outputs found
The Peculiar Periodic YSO WL 4 in ρ Ophiuchus
Full text linkWe present the discovery of 130.87 day periodic near-infrared flux variability for the Class II T Tauri star WL 4 (= 2MASS J16271848-2429059, ISO-Oph 128). Our data are from the 2MASS Calibration Point Source Working Database, and constitute 1580 observations in J, H and K_s of a field in ρ Ophiuchus used to calibrate the 2MASS All-Sky Survey. We identify a light curve for WL 4 with eclipse amplitudes of ~0.4 mag lasting more than one-quarter the period, and color variations in J-H and H-K_s, of ~0.1 mag. The long period cannot be explained by stellar rotation. We propose that WL 4 is a triple YSO system, with an inner binary orbital period of 130.87 days. We posulate that we are observing each component of the inner binary alternately being eclipsed by a circum-binary disk with respect to our line of sight. This system will be useful in investigating terrestrial zone YSO disk properties and dynamics at ~1 Myr
A Consistent Model of the Accretion Shock Region in Classical T Tauri Stars
Full text linkWe develop a consistent model of the accretion shock region in Classical T Tauri Stars (CTTSs). The initial conditions of the post-shock flow are determined by the irradiated shock precursor and the ionization state is calculated without assuming ionization equilibrium. Comparison with observations of the C IV resonance lines (λλ 1550 Å) for CTTSs indicate that the post-shock emission predicted by the model is too large, for a reasonable range of parameters. If the model is to reproduce the observations, C IV emission from CTTSs has to be dominated by pre-shock emission, for stars with moderate to large accretion rates. For stars with low accretion rates, the observations suggest a comparable contribution between the pre- and post-shock regions. These conclusions are consistent with previous results indicating that the post-shock will be buried under the stellar photosphere for moderate to large accretion rates
Measuring tiny mass accretion rates onto young brown dwarfs
Full text linkWe present low-resolution Keck I/LRIS-ADC spectra spanning from 3200–9000 Å of six young brown dwarfs in the TW Hya Association and in Upper Sco. The optical spectral types of the sample range from M7.25–M8.75, though two have near-IR spectral types of early L-dwarfs. For four brown dwarfs in our sample, the accretion luminosity is measurable only when the image is binned over large wavelength intervals. This method extends our sensivity to accretion rate down to ~10^(−13) M_☉ yr^(−1) for brown dwarfs
Stellar or Non-Stellar Light? Determining Near-Infrared Contamination in Low Mass X-ray Binaries
Full text linkLow-mass X-ray binary (LMXB) systems are comprised of a low-mass, K or M dwarflike star orbiting a compact object. Stellar black hole masses and their distributions are important inputs for binary evolution and supernova models. Currently, the main limiting factor in determining accurate black hole masses in LMXBs is the uncertainty of the orbital inclination angle due to an unknown amount of contaminating light in the near infrared. If present, this light dilutes the ellipsoidal variations of the low-mass secondary star, and thus gives the appearance of a lower orbital inclination system. It has been generally thought that the near infrared ellipsoidal light curves of these systems were relatively uncontaminated and represented primarily the light from the low-mass secondary star; however, recent disk and jet models have thrust this thinking into question. We combine our data from the Spitzer Space Telescope with our ground-based optical and near infrared data for several LMXBs to characterize and derive the amount of light contaminating the near-infrared ellipsoidal variations of the low-mass secondary star
The Dusty, Solar Type Spectroscopic Binary BD +20 307
Full text linkThe dustiest known main‐sequence star, BD +20 307, is actually a double‐lined binary with a period of 3.4202 days and a circular orbit. The system is also metal poor with [Fe/H] = −0.4. The components are late‐F and early‐G dwarfs and have a mass ratio of 1.07. The photometric period of about 3.5 days indicates that the components are synchronously rotating. The metal poor, binary nature invalidates the idea that the object is a very young single star with a warm planet‐making dust disk. Instead, the metal poor nature of the system and the lithium abundances of the components argue that the system is likely several billion years old, and so the dust disk results from the recent collision of two planetary mass rocky objects. Thus, BD +20 307 may well be the first known system with planets orbiting a close binary star
Proper Motions and Tangential Velocities for a Large Sample of Late-type M, L and T Dwarfs
Full text linkWe report proper motion measurements for 427 late‐type M, L and T dwarfs, 332 of which have been measured for the first time. Combining these new proper motions with previously published measurements yields a sample of 841 M7-T8 dwarfs. We combined parallax measurements or calculated spectrophotometric distances and computed tangential velocities for the entire sample. We find that kinematics for the full and volume‐limited 20 pc samples are consistent with those expected for the Galactic thin disk, with no significant differences between late‐type M, L, and T dwarfs. Applying an age‐velocity relation we conclude that the average kinematic age of the 20 pc sample of ultracool dwarfs is older than recent kinematic estimates and more consistent with age results calculated with population synthesis models. We isolate subsets of the entire sample, including low surface‐gravity dwarfs, unusually blue L dwarfs, and photometric outliers in J−K_s color and investigate their kinematics. We find that the spectroscopically distinct class of unusually blue L dwarfs has kinematics clearly consistent with old age, implying that high surface‐gravity and/or low metallicity may be relevant to their spectral properties. The low surface‐gravity dwarfs are kinematically younger than the overall population, and the kinematics of the red and blue ultracool dwarfs suggest ages that are younger and older than the full sample, respectively
X-ray emission in the Outer Galaxy: the Star Forming Region NGC 1893
Full text linkA key issue of the star formation process is its independence from the environmental conditions. In particular, it is not clear whether star formation in the outer Galaxy occurs in the same way as in the inner Galaxy. We present preliminary results of the analysis of a ∼440 Ks ACIS‐Chandra observation of NGC1893, a young cluster (∼3–4 Myrs), far away from the Galactic Center with the aim to study star formation in the outer region of the Galaxy and investigate the coronal properties of the cluster stars. We detect more than 1000 X‐ray sources, most of which are likely cluster members. We present also a preliminary analysis of X‐ray variability of the cluster stars
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Mapping the Radio Coronae of Cool Stars and Brown Dwarfs
Full text linkThe pulsing radio emission detected from ultracool dwarfs can be used as a powerful diagnostic of magnetic field strengths and topologies at and below the substellar boundary. Studies thus far have confirmed magnetic field strengths of 3 kG for two late M dwarfs and 1.7 kG for an L3.5 dwarf, the latter being the first confirmation of kG magnetic fields for an L dwarf. Ongoing long term monitoring of the radio pulses will also investigate the stability of the associated large‐scale magnetic fields over timescales > 1 year. We also present the preliminary results of a lengthy radio monitoring campaign of the rapidly rotating M4 star V374 Peg, with the resulting light curves phased with magnetic maps previously obtained through Zeeman Doppler Imaging. The radio emission from V374 Peg is strongly modulated by the large scale dipolar magnetic field, with two clear peaks in the radio light curve per period of rotation, occurring when the dipolar field lies in the plane of the sky. These results provide strong evidence that the electron cyclotron maser instability plays a pivotal role in the production of quiescent radio emission from V374 Peg, representing a significant departure from the accepted model of gyrosynchrotron emission as the dominant source of quiescent radio emission from active M dwarfs
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