722 research outputs found
The Arizona Radio Observatory CO Mapping Survey of Galactic Molecular Clouds: III. The Serpens Cloud in CO J=2-1 and 13CO J=2-1 Emission
We mapped 12CO and 13CO J = 2-1 emission over 1.04 square deg of the Serpens
molecular cloud with 38 arcsec spatial and 0.3 km/s spectral resolution using
the Arizona Radio Observatory Heinrich Hertz Submillimeter telescope. Our maps
resolve kinematic properties for the entire Serpens cloud. We also compare our
velocity moment maps with known positions of Young Stellar Objects (YSOs) and
1.1 mm continuum emission. We find that 12CO is self-absorbed and 13CO is
optically thick in the Serpens core. Outside of the Serpens core, gas appears
in filamentary structures having LSR velocities which are blue-shifted by up to
2 km/s relative to the 8 km/s systemic velocity of the Serpens cloud. We show
that the known Class I, Flat, and Class II YSOs in the Serpens core most likely
formed at the same spatial location and have since drifted apart. The spatial
and velocity structure of the 12CO line ratios implies that a detailed
3-dimensional radiative transfer model of the cloud will be necessary for full
interpretation of our spectral data. The starless cores region of the cloud is
likely to be the next site of star formation in Serpens.Comment: 41 pages, 15 figure
Predictable Disruption Tolerant Networks and Delivery Guarantees
This article studies disruption tolerant networks (DTNs) where each node
knows the probabilistic distribution of contacts with other nodes. It proposes
a framework that allows one to formalize the behaviour of such a network. It
generalizes extreme cases that have been studied before where (a) either nodes
only know their contact frequency with each other or (b) they have a perfect
knowledge of who meets who and when. This paper then gives an example of how
this framework can be used; it shows how one can find a packet forwarding
algorithm optimized to meet the 'delay/bandwidth consumption' trade-off:
packets are duplicated so as to (statistically) guarantee a given delay or
delivery probability, but not too much so as to reduce the bandwidth, energy,
and memory consumption.Comment: 9 page
Phase-resolved far-ultraviolet HST spectroscopy of the peculiar magnetic white dwarf RE J0317-853
We present phase resolved FUV HST FOS spectra of the rapidly rotating, highly
magnetic white dwarf RE J0317-853. Using these data, we construct a new model
for the magnetic field morphology across the stellar surface. From an expansion
into spherical harmonics, we find the range of magnetic field strengths present
is 180-800MG. For the first time we could identify an absorption feature
present at certain phases at 1160A as a ``forbidden'' 1s_0 -> 2s_0 component,
due to the combined presence of an electric and magnetic field.Comment: 15 pages including 4 figures. Accepted for publication in ApJ Letter
NLTT5306: The shortest Period Detached White Dwarf + Brown Dwarf Binary
We have spectroscopically confirmed a brown dwarf mass companion to the
hydrogen atmosphere white dwarf NLTT5306. The white dwarf's atmospheric
parameters were measured using Sloan Digital Sky Survey and X-Shooter
spectroscopy as T_eff=7756+/-35K and log(g)=7.68+/-0.08, giving a mass for the
primary of M_WD=0.44+/-0.04 M_sun, at a distance of 71+/-4 pc with a cooling
age of 710+/-50 Myr. The existence of the brown dwarf secondary was confirmed
through the near-infrared arm of the X-Shooter data and a spectral type of
dL4-dL7 was estimated using standard spectral indices. Combined radial velocity
measurements from the Sloan Digital Sky Survey, X-Shooter and the Hobby-Eberly
Telescope's High Resolution Spectrograph of the white dwarf gives a minimum
mass of 56+/-3 M_jup for the secondary, confirming the substellar nature. The
period of the binary was measured as 101.88+/-0.02 mins using both the radial
velocity data and i'-band variability detected with the INT. This variability
indicates 'day' side heating of the brown dwarf companion. We also observe
H{\alpha} emission in our higher resolution data in phase with the white dwarf
radial velocity, indicating this system is in a low level of accretion, most
likely via a stellar wind. This system represents the shortest period white
dwarf + brown dwarf binary and the secondary has survived a stage of common
envelope evolution, much like its longer period counterpart, WD0137-349. Both
systems likely represent bona-fide progenitors of cataclysmic variables with a
low mass white dwarf and a brown dwarf donor.Comment: 9 pages, 11 figures, accepted for publication in MNRA
Near-Infrared Constraints on the Presence of Warm Dust at Metal-Rich, Helium Atmosphere White Dwarfs
Here, we present near-infrared spectroscopic observations of 15 helium
atmosphere, metal-rich white dwarfs obtained at the NASA Infrared Telescope
Facility. While a connection has been demonstrated between the most highly
polluted, hydrogen atmosphere white dwarfs and the presence of warm
circumstellar dust and gas, their frequency at the helium atmosphere variety is
poorly constrained. None of our targets show excess near-infrared radiation
consistent with warm orbiting material. Adding these near-infrared constraints
to previous near- and mid-infrared observations, the frequency of warm
circumstellar material at metal-bearing white dwarfs is at least 20% for
hydrogen-dominated photospheres, but could be less than 5% for those
effectively composed of helium alone. The lower occurrence of dust disks around
helium atmosphere white dwarfs is consistent with Myr timescales for
photospheric metals in massive convection zones. Analyzing the mass
distribution of 10 white dwarfs with warm circumstellar material, we search for
similar trends between the frequency of disks and the predicted frequency of
massive planets around intermediate mass stars, but find the probability that
disk-bearing white dwarfs are more massive than average is not significant.Comment: AJ, in pres
Spectroscopic and Photometric Analysis of HS 1136+6646: A Hot Young DAO+K7 V Post-Common-Envelope, Pre-Cataclysmic Variable Binary
Extensive photometric and spectroscopic observations have been obtained for HS 1136+6646. The observations reveal a newly formed post–common-envelope binary system containing a hot ~DAO.5 primary and a highly irradiated secondary. HS 1136+6646 is the most extreme example yet of a class of short-period hot H-rich white dwarfs with K–M companion systems such as V471 Tau and Feige 24. HS 1136+6646 is a double-line spectroscopic binary showing emission lines of H i, He ii, C ii, Ca ii, and Mg ii, due in part to irradiation of the K7 V secondary by the hot white dwarf. Echelle spectra reveal the hydrogen emission lines to be double-peaked with widths of ~200 km s-1, raising the possibility that emission from an optically thin disk may also contribute. The emission lines are observed to disappear near the inferior conjunction. An orbital period of 0:83607 ± 0:00003 days has been determined through the phasing of radial velocities, emission-line equivalent widths, and photometric measurements spanning a range of 24 months. Radial velocity measurements yield an amplitude of KWD ¼ 69 ± 2 km s-1 for the white dwarf and KK7V = 115 ± 1 km s-1 for the secondary star. In addition to orbital variations, photometric measurements have also revealed a low-amplitude modulation with a period of 113.13 minutes and a semiamplitude of 0.0093 mag. These short-period modulations are possibly associated with the rotation of the white dwarf. From fits of the Balmer line profiles, the white dwarf is estimated to have an effective temperature and gravity of ~70,000 K and log g ~ 7:75, respectively. However, this optically derived temperature is difficult to reconcile with the far-UV spectrum of the Lyman line region. Far Ultraviolet Spectroscopic Explorer spectra show the presence of O vi absorption lines and a spectral energy distribution whose slope persists nearly to the Lyman limit. The extremely high temperature of the white dwarf, from both optical and UV measurements, indicates that the binary system is one of the earliest post–common-envelope objects known, having an age around 7:7 x 105 yr. Although the spectrum of the secondary star is best represented by a K7 V star, indications are that the star may be overly luminous for its mass
Transient Ionospheric Upflow Driven by Poleward Moving Auroral forms Observed During the Rocket Experiment for Neutral Upwelling 2 (RENU2) Campaign
This study examines cumulative effects of a series of poleward moving auroral forms on ion upflow and downflow. These effects are investigated using an ionospheric model with inputs derived from the Rocket Experiment for Neutral Upwelling 2 (RENU2) sounding rocket campaign. Auroral precipitation inputs are constrained by all‐sky imager brightness values resulting in significant latitudinal structuring in simulated ionospheric upflows due to transient forcing. For contrast, a case with steady forcing generates almost double the O+ upflow transport through 1,000 km when compared to poleward moving auroral form‐like structures. At high altitudes, model results show a spread in upflow response time dependent on ion mass, with molecular ions responding slower than atomic ions by several minutes. While the modeled auroral precipitation is not strong enough to accelerate ions to escape velocities, source populations available for higher‐altitude energization processes are greatly impacted by variable forcing exhibited by the RENU2 event.Key PointsImager data provide realistic transient forcing constraints for model inputs to simulate observations from a high‐latitude rocket flightTransient forcing deposits energy over a wider latitudinal region but less energy in any specific locationModeling a sequence of poleward moving auroral forms with realistic spatiotemporal variability generates significant latitudinal structuringPeer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/150562/1/grl59002.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/150562/2/grl59002-sup-0001-Text_SI-S01.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/150562/3/grl59002_am.pd
Probing Multiple Sight Lines through the SN 1006 Remnant by UV Absorption Spectroscopy
Absorption-line spectroscopy is an effective probe for cold ejecta within an
SNR, provided that suitable background UV sources can be identified. For the SN
1006 remnant we have identified four such sources, in addition to the
much-studied Schweitzer-Middleditch (SM) star. We have used STIS on HST to
obtain UV spectra of all four sources, to study "core samples" of the SN 1006
interior. The line of sight closest to the center of the SNR shell, passing
only 2.0 arcmin away, is to a V = 19.5 QSO at z = 1.026. Its spectrum shows
broad Fe II absorption lines, asymmetric with red wings broader than blue. The
similarity of these profiles to those seen in the SM star, which is 2.8 arcmin
from the center in the opposite direction, confirms the existence of a bulge on
the far side of SN 1006. The Fe II equivalent widths in the QSO spectrum are ~
50% greater than in the SM star, suggesting that somewhat more iron may be
present within SN 1006 than studies of the SM star alone have indicated, but
this is still far short of what most SNIa models require. The absorption
spectrum against a brighter z = 0.337 QSO seen at 57% of the shell radius shows
broad silicon absorption lines but no iron other than narrow, probably
interstellar lines. The cold iron expanding in this direction must be confined
within v <~ 5200 km/s, also consistent with a high-velocity bulge on the far
side only. The broad silicon lines indicate that the silicon layer has expanded
beyond this point, and that it has probably been heated by a reverse shock.
Finally, the spectra of two ~ A0V stars near the southern shell rim show no
broad or unusually strong absorption lines, suggesting that the low-ionization
ejecta are confined within 83% of the shell radius, at least at the azimuths of
these background sources.Comment: 26 pages, 8 postscript figure
- …