51 research outputs found
An Observational Limit on the Dwarf Galaxy Population of the Local Group
We present the results of an all-sky, deep optical survey for faint Local
Group dwarf galaxies. Candidate objects were selected from the second Palomar
survey (POSS-II) and ESO/SRC survey plates and follow-up observations performed
to determine whether they were indeed overlooked members of the Local Group.
Only two galaxies (Antlia and Cetus) were discovered this way out of 206
candidates. Based on internal and external comparisons, we estimate that our
visual survey is more than 77% complete for objects larger than one arc minute
in size and with a surface brightness greater than an extremely faint limit
over the 72% of the sky not obstructed by the Milky Way. Our limit of
sensitivity cannot be calculated exactly, but is certainly fainter than 25
magnitudes per square arc second in R, probably 25.5 and possibly approaching
26. We conclude that there are at most one or two Local Group dwarf galaxies
fitting our observational criteria still undiscovered in the clear part of the
sky, and a roughly a dozen hidden behind the Milky Way. Our work places the
"missing satellite problem" on a firm quantitative observational basis. We
present detailed data on all our candidates, including surface brightness
measurements.Comment: 58 pages in AJ manuscript format; some figures at slightly reduced
quality; accepted by the Astronomical Journa
Measuring streambed morphology using range imaging
River engineeringInnovative field and laboratory instrumentatio
X-rays and Protostars in the Trifid Nebula
The Trifid Nebula is a young HII region recently rediscovered as a
"pre-Orion" star forming region, containing protostars undergoing violent mass
ejections visible in optical jets as seen in images from the Infrared Space
Observatory and the Hubble Space Telescope. We report the first X-ray
observations of the Trifid nebula using ROSAT and ASCA. The ROSAT image shows a
dozen X-ray sources, with the brightest X-ray source being the O7 star, HD
164492, which provides most of the ionization in the nebula. We also identify
85 T Tauri star and young, massive star candidates from near-infrared colors
using the JHKs color-color diagram from the Two Micron All Sky Survey (2MASS).
Ten X-ray sources have counterpart near-infrared sources. The 2MASS stars and
X-ray sources suggest there are potentially numerous protostars in the young
HII region of the Trifid. ASCA moderate resolution spectroscopy of the
brightest source shows hard emission up to 10 keV with a clearly detected Fe K
line. The best model fit is a two-temperature (T = 1.2x10^6 K and 39x10^6 K)
thermal model with additional warm absorbing media. The hotter component has an
unusually high temperature for either an O star or an HII region; a typical
Galactic HII region could not be the primary source for such hot temperature
plasma and the Fe XXV line emission. We suggest that the hotter component
originates in either the interaction of the wind with another object (a
companion star or a dense region of the nebula) or from flares from deeply
embedded young stars.Comment: Accepted in ApJ (Oct, 20 issue, 2001
Distribution and Kinematics of O VI in the Galactic Halo
FUSE spectra of 100 extragalactic objects are analyzed to obtain measures of
O VI absorption along paths through the Milky Way thick disk/halo. Strong O VI
absorption over the approximate velocity range from -100 to 100 km/s reveals a
widespread but highly irregular distribution of thick disk O VI, implying the
existence of substantial amounts of hot gas with T ~ 3x10^5 K in the Milky Way
halo. Large irregularities in the distribution of the absorbing gas are found
to be similar over angular scales extending from less than one to 180 degrees,
indicating a considerable amount of small and large scale structure in the gas.
The overall distribution of Galactic O VI is not well described by a
symmetrical plane-parallel layer of patchy O VI absorption. The simplest
departure from such a model that provides a reasonable fit to the observations
is a plane-parallel patchy absorbing layer with a scale height of 2.3 kpc, and
a 0.25 dex excess of O VI in the northern Galactic polar region. The O VI
absorption has a Doppler parameter b = 30 to 99 km/s, with an average value of
60 km/s . Thermal broadening alone cannot explain the large observed profile
widths. The average O VI absorption velocities toward high latitude objects
range from -46 to 82 km/s, with a sample average of 0 km/s and a standard
deviation of 21 km/s. O VI associated with the thick disk moves both toward and
away from the plane with roughly equal frequency. A combination of models
involving the radiative cooling of hot fountain gas, the cooling of supernova
bubbles in the halo, and the turbulent mixing of warm and hot halo gases is
required to explain the presence of O VI and other highly ionized atoms found
in the halo. (abbreviated)Comment: 70 pages, single-spaced, PDF format. Bound copies of this manuscript
and two accompanying articles are available upon request. Submitted to ApJ
A VLA Search for Water Masers in Six HII Regions: Tracers of Triggered Low-Mass Star Formation
We present a search for water maser emission at 22 GHz associated with young
low-mass protostars in six HII regions -- M16, M20, NGC 2264, NGC 6357, S125,
and S140. The survey was conducted with the NRAO Very Large Array from 2000 to
2002. For several of these HII regions, ours are the first high-resolution
observations of water masers. We detected 16 water masers: eight in M16, four
in M20, three in S140, and one in NGC 2264. All but one of these were
previously undetected. No maser emission was detected from NGC 6357 or S125.
There are two principle results to our study. (1) The distribution of water
masers in M16 and M20 does not appear to be random but instead is concentrated
in a layer of compressed gas within a few tenths of a parsec of the ionization
front. (2) Significantly fewer masers are seen in the observed fields than
expected based on other indications of ongoing star formation, indicating that
the maser-exciting lifetime of protostars is much shorter in HII regions than
in regions of isolated star formation. Both of these results confirm
predictions of a scenario in which star formation is first triggered by shocks
driven in advance of ionization fronts, and then truncated approximately 10^5
years later when the region is overrun by the ionization front.Comment: 30 pages, 20 figures, 3 tables. Accepted for publication by ApJ. Full
resolution figures and PS and PDF versions with full-res figures available at
http://eagle.la.asu.edu/healy/preprints/hhc0
Cognitive Performance and Exposure to Organophosphate Flame Retardants in Children: Evidence from a Cross-Sectional Analysis of Two European Mother–Child Cohorts
The knowledge of the effects of organophosphate flame retardants on children’s neurodevelopment is limited. The purpose of the present research is to evaluate the association between exposure to organophosphate flame retardants and children’s neurodevelopment in two European cohorts involved in the Human Biomonitoring Initiative Aligned Studies. The participants were school-aged children belonging to the Odense Child Cohort (Denmark) and the PCB cohort (Slovakia). In each cohort, the children’s neurodevelopment was assessed through the Full-Scale Intelligence Quotient score of the Wechsler Intelligence Scale for Children, using two different editions. The children’s urine samples, collected at one point in time, were analyzed for several metabolites of organophosphate flame retardants. The association between neurodevelopment and each organophosphate flame retardant metabolite was explored by applying separate multiple linear regressions based on the approach of MM-estimation in each cohort. In the Danish cohort, the mean ± standard deviation for the neurodevelopment score was 98 ± 12; the geometric mean (95% confidence interval (95% CI)) of bis(1,3-dichloro-2-propyl) phosphate (BDCIPP) standardized by creatinine (crt) was 0.52 μg/g crt (95% CI = 0.49; 0.60), while that of diphenyl phosphate (DPHP) standardized by crt was 1.44 μg/g crt (95% CI = 1.31; 1.58). The neurodevelopment score showed a small, negative, statistically imprecise trend with BDCIPP standardized by crt (β = −1.30; 95%CI = −2.72; 0.11; p-value = 0.07) and no clear association with DPHP standardized by crt (β = −0.98; 95%CI = −2.96; 0.99; p-value = 0.33). The neurodevelopment score showed a negative trend with BDCIPP (β = −1.42; 95% CI = −2.70; −0.06; p-value = 0.04) and no clear association with DPHP (β = −1.09; 95% CI = −2.87; 0.68; p-value = 0.23). In the Slovakian cohort, the mean ± standard deviation for the neurodevelopment score was 81 ± 15; the geometric mean of BDCIPP standardized by crt was 0.18 μg/g crt (95% CI = 0.16; 0.20), while that of DPHP standardized by crt was 2.24 μg/g crt (95% CI = 2.00; 3.52). The association of the neurodevelopment score with BDCIPP standardized by crt was −0.49 (95%CI = −1.85; 0.87; p-value = 0.48), and with DPHP standardized by crt it was −0.35 (95%CI = −1.90; 1.20; p-value = 0.66). No clear associations were observed between the neurodevelopment score and BDCIPP/DPHP concentrations that were not standardized by crt. No clear associations were observed with bis(1-chloro-2-propyl) phosphate (BCIPP) in either cohort, due to the low detection frequency of this compound. In conclusion, this study provides only limited evidence of an inverse association between neurodevelopment and exposure to BDCIPP and DPHP. The timing of exposure and effect modification of other organophosphate flame retardant metabolites and other substances should be the subject of further investigations that address this scientific hypothesis
A Hubble Space Telescope Survey for Resolved Companions of Planetary-Nebula Nuclei
We report results of an HST "snapshot" survey aimed at finding resolved
binary companions of the central stars of Galactic planetary nebulae (PNe).
Using WF/PC and WFPC2, we searched the fields of 113 PNe for stars whose close
proximity to the central star suggests a physical association. We find 10
binary nuclei that are very likely to be physically associated, and another six
that are possible binary associations. By correcting for interstellar
extinction and placing the central stars' companions on the main sequence, we
derive distances to the objects, and thereby significantly increase the number
of PNe with reliable distances.
Comparison of our derived distances with those obtained from various
statistical methods shows that all of the latter have systematically
overestimated the distances, by factors ranging up to a factor of two or more.
We show that this error is most likely due to the fact that the properties of
our PNe with binary nuclei are systematically different from those of PNe used
heretofore to calibrate statistical methods. Specifically, our PNe tend to have
lower surface brightnesses at the same physical radius than the traditional
calibration objects. This difference may arise from a selection effect: the PNe
in our survey are typically nearby, old nebulae, whereas most of the objects
that calibrate statistical techniques are low-latitude,
high-surface-brightness, and more distant nebulae. As a result, the statistical
methods that seem to work well with samples of distant PNe, e.g., those in the
Galactic bulge or external galaxies, may not be applicable to the more diverse
population of local PNe.Comment: 37 text pages, 17 table pages, 9 figures. Accepted by Astronomical
Journal for June 1999 issu
Kepler observations of variability in B-type stars
The analysis of the light curves of 48 B-type stars observed by Kepler is
presented. Among these are 15 pulsating stars, all of which show low
frequencies characteristic of SPB stars. Seven of these stars also show a few
weak, isolated high frequencies and they could be considered as SPB/beta Cep
hybrids. In all cases the frequency spectra are quite different from what is
seen from ground-based observations. We suggest that this is because most of
the low frequencies are modes of high degree which are predicted to be unstable
in models of mid-B stars. We find that there are non-pulsating stars within the
beta Cep and SPB instability strips. Apart from the pulsating stars, we can
identify stars with frequency groupings similar to what is seen in Be stars but
which are not Be stars. The origin of the groupings is not clear, but may be
related to rotation. We find periodic variations in other stars which we
attribute to proximity effects in binary systems or possibly rotational
modulation. We find no evidence for pulsating stars between the cool edge of
the SPB and the hot edge of the delta Sct instability strips. None of the stars
show the broad features which can be attributed to stochastically-excited modes
as recently proposed. Among our sample of B stars are two chemically peculiar
stars, one of which is a HgMn star showing rotational modulation in the light
curve.Comment: 19 pages, 11 figures, 4 table
Long term photometric monitoring with the Mercator telescope. Frequencies and mode identification of variable O-B stars
Aims. We selected a large sample of O-B stars that were considered as (candidate) slowly pulsating B, beta Cep, and Maia stars after the analysis of their hipparcos data. We analysed our new seven passband geneva data collected for these stars during the first three years of scientific operations of the mercator telescope. We performed a frequency analysis for 28 targets with more than 50 high-quality measurements to improve their variability classification. For the pulsating stars, we tried both to identify the modes and to search for rotationally split modes. Methods: We searched for frequencies in all the geneva passbands and colours by using two independent frequency analysis methods and we applied a 3.6 S/N-level criterion to locate the significant peaks in the periodograms. The modes were identified by applying the method of photometric amplitudes for which we calculated a large, homogeneous grid of equilibrium models to perform a pulsational stability analysis. When both the radius and the projected rotational velocity of an object are known, we determined a lower limit for the rotation frequency to estimate the expected frequency spacings in rotationally split pulsation modes. Results: We detected 61 frequencies, among which 33 are new. We classified 21 objects as pulsating variables (7 new confirmed pulsating stars, including 2 hybrid beta Cep/SPB stars), 6 as non-pulsating variables (binaries or spotted stars), and 1 as photometrically constant. All the Maia candidates were reclassified into other variability classes. We performed mode identification for the pulsating variables for the first time. The most probable l value is 0, 1, 2, and 4 for 1, 31, 9, and 5 modes, respectively, including only 4 unambiguous identifications. For 7 stars we cannot rule out that some of the observed frequencies belong to the same rotationally split mode. For 4 targets we may begin to resolve close frequency multiplets. Based on observations collected with the p7 photometer attached to the Flemish 1.2-m mercator telescope situated at the Roque de los Muchachos observatory on La Palma (Spain). Section [see full textsee full text], including Figs. is only available in electronic form at http://www.aanda.org, and Tables 2 and 3 are only available in electronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/463/243Peer reviewe
Genetic and pharmacological inhibition of CDK9 drives neutrophil apoptosis to resolve inflammation in zebrafish in vivo
Neutrophilic inflammation is tightly regulated and subsequently resolves to limit tissue damage and promote repair. When the timely resolution of inflammation is dysregulated, tissue damage and disease results. One key control mechanism is neutrophil apoptosis, followed by apoptotic cell clearance by phagocytes such as macrophages. Cyclin-dependent kinase (CDK) inhibitor drugs induce neutrophil apoptosis in vitro and promote resolution of inflammation in rodent models. Here we present the first in vivo evidence, using pharmacological and genetic approaches, that CDK9 is involved in the resolution of neutrophil-dependent inflammation. Using live cell imaging in zebrafish with labelled neutrophils and macrophages, we show that pharmacological inhibition, morpholino-mediated knockdown and CRISPR/cas9-mediated knockout of CDK9 enhances inflammation resolution by reducing neutrophil numbers via induction of apoptosis after tailfin injury. Importantly, knockdown of the negative regulator La-related protein 7 (LaRP7) increased neutrophilic inflammation. Our data show that CDK9 is a possible target for controlling resolution of inflammation
- …