1,394 research outputs found
The Rotation of Young Low-Mass Stars and Brown Dwarfs
We review the current state of our knowledge concerning the rotation and
angular momentum evolution of young stellar objects and brown dwarfs from a
primarily observational view point. Periods are typically accurate to 1% and
available for about 1700 stars and 30 brown dwarfs in young clusters.
Discussion of angular momentum evolution also requires knowledge of stellar
radii, which are poorly known for pre-main sequence stars. It is clear that
rotation rates at a given age depend strongly on mass; higher mass stars
(0.4-1.2 M) have longer periods than lower mass stars and brown dwarfs.
On the other hand, specific angular momentum is approximately independent of
mass for low mass pre-main sequence stars and young brown dwarfs. A spread of
about a factor of 30 is seen at any given mass and age. The evolution of
rotation of solar-like stars during the first 100 Myr is discussed. A broad,
bimodal distribution exists at the earliest observable phases (1 Myr) for
stars more massive than 0.4 M. The rapid rotators (50-60% of the
sample) evolve to the ZAMS with little or no angular momentum loss. The slow
rotators continue to lose substantial amounts of angular momentum for up to 5
Myr, creating the even broader bimodal distribution characteristic of 30-120
Myr old clusters. Accretion disk signatures are more prevalent among slowly
rotating PMS stars, indicating a connection between accretion and rotation.
Disks appear to influence rotation for, at most, 5 Myr, and considerably
less than that for the majority of stars. If the dense clusters studied so far
are an accurate guide, then the typical solar-like star may have only 1
Myr for this task. It appears that both disk interactions and stellar winds are
less efficient at braking these objects.Comment: Review chapter for Protostars and Planets V. 15 page and 8 figure
Near-infrared polarimetric observations of the afterglow of GRB 000301C
Based on near-infrared polarimetric observations we constrain the degree of
linear polarization of the afterglow light of GRB 000301C to less than 30% 1.8
days after the burst.Comment: To appear in: Proc. 20th Texas Symposium on Relativistic
Astrophysics, eds. J. C. Wheeler and H. Marte
The Mass Dependence of Stellar Rotation in the Orion Nebula Cluster
We have determined new rotation periods for 404 stars in the Orion Nebula
Cluster using the Wide Field Imager attached to the MPG/ESO 2.2 m telescope on
La Silla, Chile. Mass estimates are available for 335 of these and most have M
< 0.3 M_sun. We confirm the existence of a bimodal period distribution for the
higher mass stars in our sample and show that the median rotation rate
decreases with increasing mass for stars in the range 0.1 < M <0.4 M_sun. While
the spread in angular momentum (J) at any given mass is more than a factor of
10, the majority of lower mass stars in the ONC rotate at rates approaching 30%
of their critical break-up velocity, as opposed to 5-10% for solar-like stars.
This is a consequence of both a small increase in observed specific angular
momentum (j=J/M) and a larger decrease in the critical value of j with
decreasing mass. Perhaps the most striking fact, however, is that j varies by
so little - less than a factor of two - over the interval 0.1-1.0 M_sun. The
distribution of rotation rates with mass in the ONC (age ~ 1 My) is similar in
nature to what is found in the Pleiades (age ~ 100 My). These observations
provide a significant new guide and test for models of stellar angular momentum
evolution during the proto-stellar and pre-main sequence phases.Comment: 11 pages, 3 figure
A Mid-Wisconsinan Pollen Diagram From Black Hawk County, Iowa
A pollen sequence spanning the mid-Wisconsinan Farmdalian interstadial, from 34,460 to 20,850 RCYBP (radiocarbon years before present), was recovered from a peat along a cut bank on the Wapsipinicon River in Black Hawk County, Iowa. The pollen sequence is divided into three zones. Zone I at the base of the peat is dominated by Pinus and NAP (nonarboreal pollen) believed to represent an open pine parkland. Zone II, dominated by Picea and Pinus pollen, is interpreted as the record of a closed conifer forest, and Zone III, dominated by Picea and NAP, as open, taiga-like vegetation. Changes in the pollen sequence appear to reflect the abatement and subsequent return of glacial conditions. Comparable changes are recorded in pollen diagrams from elsewhere in Iowa, Missouri, Illinois, and Kansas
The spectral type of CHS7797 - an intriguing very low mass periodic variable in the Orion Nebula Cluster
We present the spectroscopic characterization of the unusual high-amplitude
very low mass pre-main-sequence periodic variable CHS7797. This study is based
on optical medium-resolution (R=2200) spectroscopy in the 6450-8600 A range,
carried out with GMOS-GEMINI-S in March 2011. Observations of CHS7797 have been
carried out at two distinct phases of the 17.8d period, namely at maximum and
four days before maximum. Four different spectral indices were used for the
spectral classification at these two phases, all of them well-suited for
spectral classification of young and obscured late M dwarfs. In addition, the
gravity-sensitive NaI (8183/8195 A) and KI (7665/7699 A) doublet lines were
used to confirm the young age of CHS7797. From the spectrum obtained at maximum
light we derived a spectral type (SpT) of M6.05, while for the spectrum taken
four days before maximum the derived SpT is M5.75. The derived SpTs confirm
that CHS7797 has a mass in the stellar-substellar boundary mass range. In
addition, the small differences in the derived SpTs at the two observed phases
may provide indirect hints that CHS7797 is a binary system of similar mass
components surrounded by a tilted circumbinary disk, a system similar to KH15D.Comment: 6 pages, accepted for publication A&
Proper Motions of the Jets in the Region of HH 30 and HL/XZ Tau. Evidence for a Binary Exciting Source of the HH 30 Jet
We present [SII] images of the HH 30 and HL/XZ Tau region obtained at two
epochs, as well as long-slit optical spectroscopy of the HH 30 jet. We measured
proper motions of about 100-300 km/s for the HH 30 jet and counterjet, and of
about 120 km/s for the HL Tau jet. Inclination angles with respect to the plane
of the sky are 0-40 deg for the HH 30 jet and 60 deg for the HL Tau jet.
Comparison with previous observations suggests that most of the jet knots
consist of persisting structures. Also, we corroborate that the HH 30-N knots
correspond to the head of the HH 30 jet. The overall HH 30 jet structure can be
well described by a wiggling ballistic jet, arising either by the orbital
motion of the jet source around a primary or by precession of the jet axis
because of the tidal effects of a companion. In the first scenario, the orbital
period would be 53 yr and the total mass 0.25-2 solar masses. In the precession
scenario, the mass of the jet source would be 0.1-1 solar masses, the orbital
period <1 yr, and the mass of the companion less than a few times 0.01 solar
masses, thus being a substellar object or a giant exoplanet. In both scenarios
a binary system with a separation <18 AU (<0.13 arcsec) is required. Since the
radius of the flared disk observed with the HST is about 250 AU, we conclude
that this disk appears to be circumbinary rather than circumstellar, suggesting
that the search for the collimating agent of the HH 30 jet should be carried
out at much smaller scales.Comment: 42 pages, 7 figures, 5 tables. To Appear in The Astronomical Journal,
Vol. 133 No. 6 (June 2007
Laser cooling with electromagnetically induced transparency: Application to trapped samples of ions or neutral atoms
A novel method of ground state laser cooling of trapped atoms utilizes the
absorption profile of a three (or multi-) level system which is tailored by a
quantum interference. With cooling rates comparable to conventional sideband
cooling, lower final temperatures may be achieved. The method was
experimentally implemented to cool a single Ca ion to its vibrational
ground state. Since a broad band of vibrational frequencies can be cooled
simultaneously, the technique will be particularly useful for the cooling of
larger ion strings, thereby being of great practical importance for
initializing a quantum register based on trapped ions. We also discuss its
application to different level schemes and for ground state cooling of neutral
atoms trapped by a far detuned standing wave laser field.Comment: 9 pages, 13 figures, submitted to Appl Phys B 200
Physical properties of the jet from DG Tauri on sub-arcsecond scales with HST/STIS
We derive the physical properties at the base of the jet from DG Tau both
along and across the flow and as a function of velocity. We analysed seven
optical spectra of the DG Tau jet, taken with the Hubble Space Telescope
Imaging Spectrograph. The spectra were obtained by placing a long-slit parallel
to the jet axis and stepping it across the jet width. The resulting
position-velocity diagrams in optical forbidden emission lines allowed access
to plasma conditions via calculation of emission line ratios.
We find at the base of the jet high electron density, 10, and
very low ionisation, , which combine to give a total
density up to 3 10. This analysis confirms previous reports of
variations in plasma parameters along the jet, (i.e. decrease in density by
several orders of magnitude, increase of from 0.05 to a plateau at 0.7
downstream at 2 from the star). Furthermore, a spatial coincidence is
revealed between sharp gradients in the total density and supersonic velocity
jumps. This strongly suggests that the emission is caused by shock excitation.
The position-velocity diagrams indicate the presence of both fast accelerating
gas and slower, less collimated material. We derive the mass outflow rate,
, in the blue-shifted lobe in different velocity channels, that
contribute to a total of 8 4 10 M
yr. We estimate that a symmetric bipolar jet would transport at the low
and intermediate velocities probed by rotation measurements, an angular
momentum flux of 2.9 1.5 10 M yr
AU km s.
The derived properties of the DG Tau jet are demonstrated to be consistent
with magneto-centrifugal theory. However, non-stationary modelling is required
in order to explain all of the features revealed at high resolution.Comment: 16 pages, 18 figure
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