892 research outputs found
XMM-Newton observations of HD189733 during planetary transits
We report on two XMM-Newton observations of the planetary host star HD189733.
The system has a close in planet and it can potentially affect the coronal
structure via interactions with the magnetosphere. We have obtained X-ray
spectra and light curves from EPIC and RGS on board XMM-Newton which we have
analyzed and interpreted. We reduced X-ray data from primary transit and
secondary eclipse occurred in April 17th 2007 and May 18th 2009, respectively.
In the April 2007 observation only variability due to weak flares is
recognized. In 2009 HD189733 exhibited a X-ray flux always larger than in the
2007 observation. The average flux in 2009 was higher than in 2007 observation
by a factor of 45%. During the 2009 secondary eclipse we observed a softening
of the X-ray spectrum significant at level of ~3 sigma. Further, we observed
the most intense flare recorded at either epochs. This flare occurred 3 ks
after the end of the eclipse.The flare decay shows several minor ignitions
perhaps linked to the main event and hinting for secondary loops that emit
triggered by the main loop. Magneto-Hydro-Dynamical (MHD) simulations show that
the magnetic interaction between planet and star enhances the density and the
magnetic field in a region comprised between the planet and the star because of
their relative orbital/rotation motion. X-ray observations and model
predictions are globally found in agreement, despite the quite simple MHD model
and the lack of precise estimate of parameters including the alignment and the
intensity of stellar and planetary magnetic fields. Future observations should
confirm or disprove this hypothesis, by determining whether flares are
systematically recurring in the light curve at the same planetary phase.Comment: Accepted for publication on The Astrophysical Journa
A Combined Spitzer and Chandra Survey of Young Stellar Objects in the Serpens Cloud Core
We present Spitzer and Chandra observations of the nearby (~260 pc) embedded
stellar cluster in the Serpens Cloud Core. We observed, using Spitzer's IRAC
and MIPS instruments, in six wavelength bands from 3 to 70 , to detect
thermal emission from circumstellar disks and protostellar envelopes, and to
classify stars using color-color diagrams and spectral energy distributions
(SEDs). These data are combined with Chandra observations to examine the
effects of circumstellar disks on stellar X-ray properties. Young diskless
stars were also identified from their increased X-ray emission. We have
identified 138 YSOs in Serpens: 22 class 0/I, 16 flat spectrum, 62 class II, 17
transition disk, and 21 class III stars; 60 of which exhibit X-ray emission.
Our primary results are the following: 1.) ten protostars detected previously
in the sub-millimeter are detected at lambda < 24 microns, seven at lambda < 8
microns, 2.) the protostars are more closely grouped than more evolved YSOs
(median separation : ~0.024 pc, and 3.) the luminosity and temperature of the
X-ray emitting plasma around these YSOs does not show any significant
dependence on evolutionary class. We combine the infrared derived values of AK
and X-ray values of NH for 8 class III objects and find that the column density
of hydrogen gas per mag of extinctions is less than half the standard
interstellar value, for AK > 1. This may be the result of grain growth through
coagulation and/or the accretion of volatiles in the Serpens cloud core.Comment: 69 pages, 16 figures, accepted to ApJ. Higher Resolution Figures at:
http://www.cfa.harvard.edu/~ewinston
Obesity and the risk of gallbladder cancer: a meta-analysis
We performed a meta-analysis of studies of the association between excess body weight and risk of gallbladder cancer identified from MEDLINE and EMBASE databases from 1966 to February 2007 and the references of retrieved articles. A random-effects model was used to combine results from eight cohort studies and three case–control studies, with a total of 3288 cases. Compared with individuals of ‘normal weight', the summary relative risk of gallbladder cancer for those who were overweight or obese was 1.15 (95% CI, 1.01–1.30) and 1.66 (95% CI, 1.47–1.88) respectively. The association with obesity was stronger for women (relative risk, 1.88; 95% CI, 1.66–2.13) than for men (relative risk, 1.35; 95% CI, 1.09–1.68). There was no statistically significant heterogeneity among the results of individual studies. This meta-analysis confirms the association between excess body weight and risk of gallbladder cancer
Young Stellar Object Variability (YSOVAR): Long Timescale Variations in the Mid-Infrared
The YSOVAR (Young Stellar Object VARiability) Spitzer Space Telescope
observing program obtained the first extensive mid-infrared (3.6 & 4.5 um)
time-series photometry of the Orion Nebula Cluster plus smaller footprints in
eleven other star-forming cores (AFGL490, NGC1333, MonR2, GGD 12-15, NGC2264,
L1688, Serpens Main, Serpens South, IRAS 20050+2720, IC1396A, and Ceph C).
There are ~29,000 unique objects with light curves in either or both IRAC
channels in the YSOVAR data set. We present the data collection and reduction
for the Spitzer and ancillary data, and define the "standard sample" on which
we calculate statistics, consisting of fast cadence data, with epochs about
twice per day for ~40d. We also define a "standard sample of members",
consisting of all the IR-selected members and X-ray selected members. We
characterize the standard sample in terms of other properties, such as spectral
energy distribution shape. We use three mechanisms to identify variables in the
fast cadence data--the Stetson index, a chi^2 fit to a flat light curve, and
significant periodicity. We also identified variables on the longest timescales
possible of ~6 years, by comparing measurements taken early in the Spitzer
mission with the mean from our YSOVAR campaign. The fraction of members in each
cluster that are variable on these longest timescales is a function of the
ratio of Class I/total members in each cluster, such that clusters with a
higher fraction of Class I objects also have a higher fraction of long-term
variables. For objects with a YSOVAR-determined period and a [3.6]-[8] color,
we find that a star with a longer period is more likely than those with shorter
periods to have an IR excess. We do not find any evidence for variability that
causes [3.6]-[4.5] excesses to appear or vanish within our data; out of members
and field objects combined, at most 0.02% may have transient IR excesses.Comment: Accepted to AJ; 38 figures, 93 page
Tunable local polariton modes in semiconductors
We study the local states within the polariton bandgap that arise due to deep
defect centers with strong electron-phonon coupling. Electron transitions
involving deep levels may result in alteration of local elastic constants. In
this case, substantial reversible transformations of the impurity polariton
density of states occur, which include the appearance/disappearance of the
polariton impurity band, its shift and/or the modification of its shape. These
changes can be induced by thermo- and photo-excitation of the localized
electron states or by trapping of injected charge carriers. We develop a simple
model, which is applied to the center in . Further possible
experimental realizations of the effect are discussed.Comment: 7 pages, 3 figure
Chandra Observations of Associates of Car: I. Luminosities
The region around the Car nebula has three OB associations, which
contain a Wolf-Rayet star and several massive O3 stars. An early Chandra ACIS-I
image was centered on Car and includes Trumpler 16 and part of Trumpler
14. The Chandra image confirms the well-known result that O and very early B
stars are X-ray sources with L 10 L over an
X-ray luminosity range of about 100. Two new anomalously strong X-ray sources
have been found among the hot star population, Tr 16-244, a heavily-reddened O3
I star, and Tr 16-22, a heavily-reddened O8.5 V star. Two stars have an
unusually large L/L: HD 93162, a Wolf-Rayet star (and possible
binary), and Tr 16-22, a possible colliding wind binary In addition, a
population of sources associated with cool stars is detected. In the
color-magnitude diagram, these X-ray sources sit above the sequence of field
stars in the Carina arm. The OB stars are on average more X-ray luminous than
the cool star X-ray sources. X-ray sources among A stars have similar X-ray
luminosities to cooler stars, and may be due to cooler companions. Upper limits
are presented for B stars which are not detected in X-rays. These upper limits
are also the upper limits for any cool companions which the hot stars may have.
Hardness ratios are presented for the most luminous sources in bands 0.5 to 0.9
keV, 0.9 to 1.5 keV, and 1.5 to 2.04 kev. The available information on the
binary nature of the hot stars is discussed, but binarity does not correlate
with X-ray strength in a simple way.Comment: accepted by Ap
The VIMOS Public Extragalactic Redshift Survey (VIPERS): On the correct recovery of the count-in-cell probability distribution function
We compare three methods to measure the count-in-cell probability density
function of galaxies in a spectroscopic redshift survey. From this comparison
we found that when the sampling is low (the average number of object per cell
is around unity) it is necessary to use a parametric method to model the galaxy
distribution. We used a set of mock catalogues of VIPERS, in order to verify if
we were able to reconstruct the cell-count probability distribution once the
observational strategy is applied. We find that in the simulated catalogues,
the probability distribution of galaxies is better represented by a Gamma
expansion than a Skewed Log-Normal. Finally, we correct the cell-count
probability distribution function from the angular selection effect of the
VIMOS instrument and study the redshift and absolute magnitude dependency of
the underlying galaxy density function in VIPERS from redshift to .
We found very weak evolution of the probability density distribution function
and that it is well approximated, independently from the chosen tracers, by a
Gamma distribution.Comment: 14 pages, 11 figures, 2 table
The VIMOS Public Extragalactic Survey (VIPERS): First Data Release of 57 204 spectroscopic measurements
We present the first Public Data Release (PDR-1) of the VIMOS Public
Extragalactic Survey (VIPERS). It comprises 57 204 spectroscopic measurements
together with all additional information necessary for optimal scientific
exploitation of the data, in particular the associated photometric measurements
and quantification of the photometric and survey completeness. VIPERS is an ESO
Large Programme designed to build a spectroscopic sample of ' 100 000 galaxies
with iAB < 22.5 and 0.5 < z < 1.5 with high sampling rate (~45%). The survey
spectroscopic targets are selected from the CFHTLS-Wide five-band catalogues in
the W1 and W4 fields. The final survey will cover a total area of nearly 24
deg2, for a total comoving volume between z = 0.5 and 1.2 of ~4x10^7
h^(-3)Mpc^3 and a median galaxy redshift of z~0.8. The release presented in
this paper includes data from virtually the entire W4 field and nearly half of
the W1 area, thus representing 64% of the final dataset. We provide a detailed
description of sample selection, observations and data reduction procedures; we
summarise the global properties of the spectroscopic catalogue and explain the
associated data products and their use, and provide all the details for
accessing the data through the survey database (http://vipers.inaf.it) where
all information can be queried interactively.Comment: Accepted for publication in A&A Added and/or replaced some figure,
added section on DataBase interface, expaned Introductio
The VIMOS Public Extragalactic Redshift Survey (VIPERS): A quiescent formation of massive red-sequence galaxies over the past 9 Gyr
We explore the evolution of the Colour-Magnitude Relation (CMR) and
Luminosity Function (LF) at 0.4<z<1.3 from the VIMOS Public Extragalactic
Redshift Survey (VIPERS) using ~45,000 galaxies with precise spectroscopic
redshifts down to i'_AB<22.5 over ~10.32 deg^2 in two fields. From z=0.5 to
z=1.3 the LF and CMR are well defined for different galaxy populations and
M^*_B evolves by ~1.04(1.09)+/-0.06(0.10) mag for the total (red) galaxy
sample. We compare different criteria for selecting early-type galaxies (ETGs):
(1) fixed cut in rest-frame (U-V) colours, (2) evolving cut in (U-V) colours,
(3) rest-frame (NUV-r')-(r'-K) colour selection, and (4) SED classification.
Regardless of the method we measure a consistent evolution of the red-sequence
(RS). Between 0.4<z<1.3 we find a moderate evolution of the RS intercept of
Delta(U-V)=0.28+/-0.14 mag, favouring exponentially declining star formation
(SF) histories with SF truncation at 1.7<=z<=2.3. Together with the rise in the
ETG number density by 0.64 dex since z=1, this suggests a rapid build-up of
massive galaxies (M>10^11 M_sun) and expeditious RS formation over a short
period of ~1.5 Gyr starting before z=1. This is supported by the detection of
ongoing SF in ETGs at 0.9<z<1.0, in contrast with the quiescent red stellar
populations of ETGs at 0.5<z<0.6. There is an increase in the observed CMR
scatter with redshift, two times larger than in galaxy clusters and at variance
with theoretical models. We discuss possible physical mechanisms that support
the observed evolution of the red galaxy population. Our findings point out
that massive galaxies have experienced a sharp SF quenching at z~1 with only
limited additional merging. In contrast, less-massive galaxies experience a mix
of SF truncation and minor mergers which build-up the low- and
intermediate-mass end of the CMR.Comment: 27 pages, 21 figures, accepted for publication in A&
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