124 research outputs found
An Automated tool to detect variable sources in the Vista Variables in the VÃa Láctea Survey. The VVV Variables (V^4) catalog of tiles d001 and d002
27 pages, 19 figuresTime-varying phenomena are one of the most substantial sources of astrophysical information, and their study has led to many fundamental discoveries in modern astronomy. We have developed an automated tool to search for and analyze variable sources in the near-infrared K s band using the data from the VISTA Variables in the VÃa Láctea (VVV) ESO Public Large Survey. This process relies on the characterization of variable sources using different variability indices calculated from time series generated with point-spread function (PSF) photometry of sources under analysis. In particular, we used two main indices, the total amplitude and the eta index η, to identify variable sources. Once the variable objects are identified, periods are determined with generalized Lomb-Scargle periodograms and the information potential metric. Variability classes are assigned according to a compromise between comparisons with VVV templates and the period of the variability. The automated tool is applied on VVV tiles d001 and d002 and led to the discovery of 200 variable sources. We detected 70 irregular variable sources and 130 periodic ones. In addition, nine open-cluster candidates projected in the region are analyzed, and the infrared variable candidates found around these clusters are further scrutinized by cross-matching their locations against emission star candidates from VPHAS+ survey H α color cuts.Peer reviewedFinal Accepted Versio
Exploring Terrestrial Planet Formation in the TW Hydrae Association
Spitzer Space Telescope infrared measurements are presented for 24 members of the TW Hydrae association (TWA). High signal-to-noise 24-micron (um) photometry is presented for all of these stars, including 20 stars that were not detected by IRAS. Among these 20 stars, only a single object, TWA 7, shows excess emission at 24um and at the level of only 40% above the star's photosphere. TWA 7 also exhibits a strong 70um excess that is a factor of 40 brighter than the stellar photosphere at this wavelength. At 70um, an excess of similar magnitude is detected for TWA 13, though no 24um excess was detected for this binary. For the 18 stars that failed to show measurable IR excesses, the sensitivity of the current 70um observations does not rule out substantial cool excesses at levels 10-40x above their stellar continua. Measurements of two T Tauri stars, TW Hya and Hen 6-300, confirm that their spectacular IR spectral energy distributions (SEDs) do not turn over even by 160um, consistent with the expectation for their active accretion disks. In contrast, the Spitzer data for the luminous planetary debris systems in the TWA, HD 98800B and HR 4796A, are consistent with single-temperature blackbody SEDs. The major new result of this study is the dramatic bimodal distribution found for the association in the form of excess emission at a wavelength of 24um, indicating negligible amounts of warm (>100 K) dust and debris around 20 of 24 stars in this group of very young stars. This bimodal distribution is especially striking given that the four stars in the association with strong IR excesses are >100x brighter at 24um than their photospheres
Association of caregiver quality of care with neurocognitive outcomes in HIV-affected children aged 2–5 years in Uganda
Abstract: Children affected by HIV are at increased risk of developmental and neuropsychological disturbances due to direct effects of HIV on the brain and direct effects associated with living in poverty. Caregivers can play an important role, through quality caregiving, in mitigating the negative effect of these stressors. This study used baseline data from an ongoing caregiver training intervention trial to evaluate the association between quality of caregiver-child interactions and neurocognitive outcomes in rural HIV-infected and HIV-exposed but uninfected children in Uganda. We also assessed the extent to which caregiver distress moderated this relationship. Data on 329 caregiver-child dyads were collected between March 2012 and July 2014, when the children were between 2 and 5 years of age. Child outcomes include the Mullen Scales of Early Learning to assess general cognitive ability and the Color Object Association Test to assess immediate memory and total recall. Caregiving quality was assessed using the Home Observation for the Measurement of the Environment (HOME) total and subscale scores. Caregiver distress was assessed using the Hopkins Symptom Checklist. General linear regression models assessed the association between the HOME total and subscale scores and child outcomes, with interaction terms used to test moderation by caregiver distress. Total HOME scores were positively and significantly associated with Mullen scores of cognitive ability; HOME acceptance subscale scores were positively and significantly associated with immediate recall scores. No other associations were statistically significant. As hypothesized, there is a strong association between the HOME and Mullen scores of cognitive ability in our study population, such that children who were assessed as living in environments with more stimulation also presented with a higher level of general neurocognitive development. Our results support the view of program guidance for HIV-affected children that suggest family-oriented care with emphasis on parent-child relationships for optimal child development
Near-Infrared Variability in Dusty White Dwarfs: Tracing the Accretion of Planetary Material
The inwards scattering of planetesimals towards white dwarfs is expected to be a stochastic process with variability on human time-scales. The planetesimals tidally disrupt at the Roche radius, producing dusty debris detectable as excess infrared emission. When sufficiently close to the white dwarf, this debris sublimates and accretes on to the white dwarf and pollutes its atmosphere. Studying this infrared emission around polluted white dwarfs can reveal how this planetary material arrives in their atmospheres. We report a near-infrared monitoring campaign of 34 white dwarfs with infrared excesses with the aim to search for variability in the dust emission. Time series photometry of these white dwarfs from the United Kingdom Infrared Telescope (Wide Field Camera) in the J, H and K bands were obtained over baselines of up to three years. We find no statistically significant variation in the dust emission in all three near-infrared bands. Specifically, we can rule out variability at ∼ 1.3% for the 13 white dwarfs brighter than 16th mag in K band, and at ∼ 10% for the 32 white dwarfs brighter than 18th mag over time-scales of three years. Although to date two white dwarfs, SDSS J095904.69−020047.6 and WD 1226+110, have shown K band variability, in our sample we see no evidence of new K band variability at these levels. One interpretation is that the tidal disruption events which lead to large variabilities are rare, occur on short time-scales
Near-infrared variability in dusty white dwarfs: tracing the accretion of planetary material
The inwards scattering of planetesimals towards white dwarfs is expected to
be a stochastic process with variability on human time-scales. The
planetesimals tidally disrupt at the Roche radius, producing dusty debris
detectable as excess infrared emission. When sufficiently close to the white
dwarf, this debris sublimates and accretes on to the white dwarf and pollutes
its atmosphere. Studying this infrared emission around polluted white dwarfs
can reveal how this planetary material arrives in their atmospheres. We report
a near-infrared monitoring campaign of 34 white dwarfs with infrared excesses
with the aim to search for variability in the dust emission. Time series
photometry of these white dwarfs from the United Kingdom Infrared Telescope
(Wide Field Camera) in the J, H and K bands were obtained over baselines of up
to three years. We find no statistically significant variation in the dust
emission in all three near-infrared bands. Specifically, we can rule out
variability at ~1.3% for the 13 white dwarfs brighter than 16th mag in K band,
and at ~10% for the 32 white dwarfs brighter than 18th mag over time-scales of
three years. Although to date two white dwarfs, SDSS J095904.69-020047.6 and WD
1226+110, have shown K band variability, in our sample we see no evidence of
new K band variability at these levels. One interpretation is that the tidal
disruption events which lead to large variabilities are rare, occur on short
time-scales, and after a few years the white dwarfs return to being stable in
the near-infrared.Comment: 14 pages + supplementary figures, 9 figures, accepted for publication
in MNRA
Shallow Ultraviolet Transits of WD 1145+017
WD 1145+017 is a unique white dwarf system that has a heavily polluted
atmosphere, an infrared excess from a dust disk, numerous broad absorption
lines from circumstellar gas, and changing transit features, likely from
fragments of an actively disintegrating asteroid. Here, we present results from
a large photometric and spectroscopic campaign with Hubble, Keck , VLT,
Spitzer, and many other smaller telescopes from 2015 to 2018. Somewhat
surprisingly, but consistent with previous observations in the u' band, the UV
transit depths are always shallower than those in the optical. We develop a
model that can quantitatively explain the observed "bluing" and the main
findings are: I. the transiting objects, circumstellar gas, and white dwarf are
all aligned along our line of sight; II. the transiting object is blocking a
larger fraction of the circumstellar gas than of the white dwarf itself.
Because most circumstellar lines are concentrated in the UV, the UV flux
appears to be less blocked compared to the optical during a transit, leading to
a shallower UV transit. This scenario is further supported by the strong
anti-correlation between optical transit depth and circumstellar line strength.
We have yet to detect any wavelength-dependent transits caused by the
transiting material around WD 1145+017.Comment: 16 pages, 11 figures, 6 tables, ApJ, in pres
New chondritic bodies identified in eight oxygen-bearing white dwarfs
We present observations and analyses of eight white dwarf stars that have
accreted rocky material from their surrounding planetary systems. The spectra
of these helium-atmosphere white dwarfs contain detectable optical lines of all
four major rock-forming elements (O, Mg, Si, Fe). This work increases the
sample of oxygen-bearing white dwarfs with parent body composition analyses by
roughly thirty-three percent. To first order, the parent bodies that have been
accreted by the eight white dwarfs are similar to those of chondritic
meteorites in relative elemental abundances and oxidation states. Seventy-five
percent of the white dwarfs in this study have observed oxygen excesses
implying volatiles in the parent bodies with abundances similar to those of
chondritic meteorites. Three white dwarfs have oxidation states that imply more
reduced material than found in CI chondrites, indicating the possible detection
of Mercury-like parent bodies, but are less constrained. These results
contribute to the recurring conclusion that extrasolar rocky bodies closely
resemble those in our solar system, and do not, as a whole, yield unusual or
unique compositions.Comment: Accepted for publication in ApJ. 7 Figures, 7 Table
Infrared emission towards 11 years after outburst: Properties of the circumstellar dust
Detailed models are presented for the late epoch mid infrared (MIR) emission
from collisionally heated grains in the shocked circumstellar gas around SN
1987A. Thermal dust emission from a region of moderate density interior to the
thick inner ring seen with the Hubble Space Telescope (HST) is found to be a
natural explanation for the MIR spectral energy distribution measured by
ISOCAM. The MIR-spectrum can be reproduced by a mixture of silicate-iron or
silicate-graphite grains or by a composition of pure graphite grains. A
composition of pure iron grains on the other hand can be excluded and a pure
silicate composition does not seem to be very likely. The dust-to-gas ratio in
the interaction zone is ~0.01%, an order of magnitude lower than estimates for
dust abundances in the winds of red supergiant (RSG) stars in the LMC. This low
dust abundance can be accounted for by a combination of evaporation through the
UV-flash from the supernova outburst and subsequent sputtering in the shocked
gas. For this explanation to hold, dust in the pre-supernova circumstellar
medium (CSM) would have to have been predominantly composed of grains other
than graphite, with a maximum size smaller than ~0.1 microns
Near-Infrared Photometry of Carbon Stars
Near-infrared, JHKL, photometry of 239 Galactic carbon-rich variable stars is
presented and discussed. From these and published data the stars were
classified as Mira or non-Mira variables and amplitudes and pulsation periods,
ranging from 222 to 948 days for the Miras, were determined for most of them. A
comparison of the colour and period relations with those of similar stars in
the Large Magellanic Cloud indicates minor differences, which may be the
consequence of sample selection effects. Apparent bolometric magnitudes were
determined by combining the mean JHKL fluxes with mid-infrared photometry from
IRAS and MSX. Then, using the Mira period luminosity relation to set the
absolute magnitudes, distances were determined -- to greater accuracy than has
hitherto been possible for this type of star. Bolometric corrections to the K
magnitude were calculated and prescriptions derived for calculating these from
various colours. Mass-loss rates were also calculated and compared to values in
the literature.
Approximately one third of the C-rich Miras and an unknown fraction of the
non-Miras exhibit apparently random obscuration events that are reminiscent of
the phenomena exhibited by the hydrogen deficient RCB stars. The underlying
cause of this is unclear, but it may be that mass loss, and consequently dust
formation, is very easily triggered from these very extended atmospheres.Comment: 35 pages, 21 figs, accepted for publication in MNRAS. Large data
table will be available on-line onl
Formation and Evolution of Planetary Systems (FEPS): Properties of Debris Dust around Solar-type Stars
We present Spitzer photometric (IRAC and MIPS) and spectroscopic (IRS low
resolution) observations for 314 stars in the Formation and Evolution of
Planetary Systems (FEPS) Legacy program. These data are used to investigate the
properties and evolution of circumstellar dust around solar-type stars spanning
ages from approximately 3 Myr to 3 Gyr. We identify 46 sources that exhibit
excess infrared emission above the stellar photosphere at 24um, and 21 sources
with excesses at 70um. Five sources with an infrared excess have
characteristics of optically thick primordial disks, while the remaining
sources have properties akin to debris systems. The fraction of systems
exhibiting a 24um excess greater than 10.2% above the photosphere is 15% for
ages < 300 Myr and declines to 2.7% for older ages. The upper envelope to the
70um fractional luminosity appears to decline over a similar age range. The
characteristic temperature of the debris inferred from the IRS spectra range
between 60 and 180 K, with evidence for the presence of cooler dust to account
for the strength of the 70um excess emission. No strong correlation is found
between dust temperature and stellar age. Comparison of the observational data
with disk models containing a power-law distribution of silicate grains suggest
that the typical inner disk radius is > 10 AU. Although the interpretation is
not unique, the lack of excess emission shortwards of 16um and the relatively
flat distribution of the 24um excess for ages <300~Myr is consistent with
steady-state collisional models.Comment: 85 pages, 18 figures, 4 tables; accepted for publication in ApJ
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