2,338 research outputs found
Another Faint UV Object Associated with a Globular Cluster X-Ray Source: The Case of M92
The core of the metal poor Galactic Globular Cluster M92 (NGC 6341) has been
observed with WFPC2 on the Hubble Space Telescope through visual, blue and
mid-UV filters in a program devoted to study the evolved stellar population in
a selected sample of Galactic Globular Clusters. In the UV color magnitude diagram we have discovered a faint `UV-dominant'
object. This star lies within the error box of a Low Luminosity Globular
Cluster X-ray source (LLGCX) recently found in the core of M92. The properties
of the UV star discovered in M92 are very similar to those of other UV stars
found in the core of some clusters (M13, 47 Tuc, M80, etc)---all of them are
brighter in the UV than in the visible and are located in the vicinity of a
LLGCX. We suggest that these stars are a new sub-class of cataclysmic
variables.Comment: 21 pages, 4 figures. Astrophysical journal in pres
Infrared Candidates for the Intense Galactic X-ray Source GX 17+2
We present new astrometric solutions and infrared Hubble Space Telescope
observations of GX 17+2 (X1813-140), one of the brightest X-ray sources on the
celestial sphere. Despite 30 years of intensive study, and the existence of a
strong radio counterpart with a sub-arcsecond position, the object remains
optically unidentified. The observed X-ray characteristics strongly suggest
that it is a so-called "Z-source," the rare but important category that
includes Sco X-1 and Cyg X-2. Use of the USNO-A2.0 catalog enables us to
measure the position of optical and infrared objects near the radio source to
sub-arcsecond precision within the International Celestial Reference Frame, for
direct comparison with the radio position, which we also recompute using modern
calibrators. With high confidence we eliminate the V~17.5 star NP Ser, often
listed as the probable optical counterpart of the X-ray source, as a candidate.
Our HST NICMOS observations show two faint objects within our 0.5" radius 90%
confidence error circle. Even the brighter of the two, Star A, is far fainter
than expected (H~19.8), given multiple estimates of the extinction in this
field and our previous understanding of Z sources, but it becomes the best
candidate for the counterpart of GX 17+2. The probability of a chance
coincidence of an unrelated faint object on the radio position is high.
However, if the true counterpart is not Star A, it is fainter still, and our
conclusion that the optical counterpart is surprisingly underluminous is but
strengthened.Comment: 15 pages including 3 figures and 3 tables. Accepted for publication
in The Astrophysical Journa
Diagrams for heat kernel expansions
A diagramatic heat kernel expansion technique is presented. The method is
especially well suited to the small-derivative expansion of the heat kernel,
but it can also be used to reproduce the results obtained by the approach known
as covariant perturbation theory. The new technique gives an expansion for the
heat kernel at coincident points. It can also be used to obtain the derivative
of the heat kernel and this is useful for evaluating the expectation values of
the stress-energy tensor.Comment: 17 pages, 4 figures, ReVTe
Photoevaporation and close encounters: how the environment around Cygnus OB2 affects the evolution of protoplanetary disks
In our Galaxy, star formation occurs in a variety of environments, with a
large fraction of stars formed in clusters hosting massive stars. OB stars have
an important feedback on the evolution of protoplanetary disks around nearby
young stars and likely on the process of planet formation occurring in them.
The nearby massive association Cygnus OB2 is an outstanding laboratory to study
this feedback. It is the closest massive association to our Sun, and hosts
hundreds of massive stars and thousands of low mass members. In this paper, we
analyze the spatial variation of the disk fraction in Cygnus OB2 and we study
its correlation with the local values of Far and Extreme ultraviolet radiation
fields and the local stellar surface density. We present definitive evidence
that disks are more rapidly dissipated in the regions of the association
characterized by intense local UV field and large stellar density. In
particular, the FUV radiation dominates disks dissipation timescales in the
proximity (i.e. within 0.5 pc) of the O stars. In the rest of the association,
EUV photons potentially induce a significant mass loss from the irradiated
disks across the entire association, but the efficiency of this process is
reduced at increasing distances from the massive stars due to absorption by the
intervening intracluster material. We find that disk dissipation due to close
stellar encounters is negligible in Cygnus OB2, and likely to have affected 1%
or fewer of the stellar population. Disk dissipation is instead dominated by
photoevaporation. We also compare our results to what has been found in other
young clusters with different massive populations, concluding that massive
associations like Cygnus OB2 are potentially hostile to protoplanetary disks,
but that the environments where disks can safely evolve in planetary systems
are likely quite common in our Galaxy.Comment: Accepted for publication in ApJS as part of the special issue on the
Chandra Cygnus OB2 Legacy Projec
The Malcolm Boat (38CH803): Discovery, Stabilization, Excavation, and Preservation of an Historic Sea Going Small Craft in the Ashley River, Charleston County, South Carolina
The following report details the results of an investigation of the remains of a small historic sailing craft, The Malcolm Boat (38CH803), discovered in a mud bank of the Ashley River in 1985. The investigation, conducted in June of 1992, with partial funding support from the South Carolina Department of Archives and History, revealed that the vessel was a small ocean-going hull dating to the last quarter of the eighteenth century and the first quarter of the nineteenth. The analysis presented discusses the vessel\u27s age, method of construction and function as a coastal or possibly inter-islander trader, and places the vessel within a regional maritime historical context. Historical context is provided in the form of the background history of shipbuilding in South Carolina and a preliminary typology of local small craft. Methods of site stabilization for intertidal zone sites are discussed with recommendations for future work in this new area of investigation in the state.https://scholarcommons.sc.edu/archanth_books/1194/thumbnail.jp
Ultraviolet Emission Line Ratios of Cataclysmic Variables
We present a statistical analysis of the ultraviolet emission lines of
cataclysmic variables (CVs) based on ultraviolet spectra of 20
sources extracted from the International Ultraviolet Explorer Uniform Low
Dispersion Archive. These spectra are used to measure the emission line fluxes
of N V, Si IV, C IV, and He II and to construct diagnostic flux ratio diagrams.
We investigate the flux ratio parameter space populated by individual CVs and
by various CV subclasses (e.g., AM Her stars, DQ Her stars, dwarf novae,
nova-like variables). For most systems, these ratios are clustered within a
range of decade for log Si IV/C IV and log He II/C IV
and decades for log N V/C IV . These
ratios are compared to photoionization and collisional ionization models to
constrain the excitation mechanism and the physical conditions of the
line-emitting gas. We find that the collisional models do the poorest job of
reproducing the data. The photoionization models reproduce the Si IV/C IV line
ratios for some shapes of the ionizing spectrum, but the predicted N V/C IV
line ratios are simultaneously too low by typically decades. Worse,
for no parameters are any of the models able to reproduce the observed He II/C
IV line ratios; this ratio is far too small in the collisional and scattering
models and too large by typically decades in the photoionization
models.Comment: LaTeX format, uses aaspp4.sty, 28 pages, 11 Postscript figures,
accepted for publication in The Astrophysical Journal 10/16/9
Are There Age Spreads in Star Forming Regions?
A luminosity spread at a given effective temperature is ubiquitously seen in
the Hertzsprung-Russell (HR) diagrams of young star forming regions and often
interpreted in terms of a prolonged period (>=10 Myr) of star formation. I
review the evidence that the observed luminosity spreads are genuine and not
caused by astrophysical sources of scatter. I then address whether the
luminosity spreads necessarily imply large age spreads, by comparing HR diagram
ages with ages from independent clocks such as stellar rotation rate, the
presence of circumstellar material and lithium depletion. I argue that whilst
there probably is a true luminosity dispersion, there is little evidence to
support age spreads larger than a few Myr. This paradox could be resolved by
brief periods of rapid accretion during the class I pre main-sequence phase.Comment: To appear in the proceedings of JENAM10: Star Clusters in the Era of
Large Surveys, 8 page
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