148 research outputs found
Pediatric rheumatology: addressing the transition to adult-orientated health care.
The transition from pediatric to adult health care is often a challenging process due to multiple interwoven complexities, especially for children with chronic medical conditions. Health care transition (HCT) is a process of moving from a pediatric to an adult model of health care with or without a transfer to a new clinician. This paper focuses on what is known about HCT for youth and young adults (Y/YA) with rheumatic diseases within a larger context of HCT recommendations. HCT barriers for youth, families, and providers and current evidence for a structured HCT processes are reviewed. Practical advice is offered on how to approach transition for Y/YA, what tools are available to assist in a successful transition process, and what are the areas of future research that are needed to improve the HCT evidence base
Discovery of seven T Tauri stars and a brown dwarf candidate in the nearby TW Hydrae Association
We report the discovery of five T Tauri star systems, two of which are
resolved binaries, in the vicinity of the nearest known region of recent star
formation, the TW Hydrae Association. The newly discovered systems display the
same signatures of youth (namely high X-ray flux, large Li abundance and strong
chromospheric activity) and the same proper motion as the original five
members. These similarities firmly establish the group as a bona fide T Tauri
association, unique in its proximity to Earth and its complete isolation from
any known molecular clouds.
At an age of ~10 Myr and a distance of ~50 pc, the association members are
excellent candidates for future studies of circumstellar disk dissipation and
the formation of brown dwarfs and planets. Indeed, as an example, our speckle
imaging revealed a faint, very likely companion 2" north of CoD-33 7795 (TWA
5). Its color and brightness suggest a spectral type ~M8.5 which, at an age of
~10^7 years, implies a mass ~20 M(Jupiter).Comment: 6 pages, 4 figures and 1 table. AAS LaTeX aas2pp4.sty. To be
published in Ap
Determining the Ages of Nearby A-Stars with Long-Baseline Interferometry
We determine the age of 7 stars in the Ursa Major moving group using a novel method that models the fundamental parameters of rapidly rotating A-stars based on interferometric observations and literature photometry and compares these parameters (namely, radius, luminosity, and rotation velocity) with evolution models that account for rotation. We find these stars to be coeval, thus providing an age estimate for the moving group and validating this technique. With this technique validated, we determine the age of the rapidly rotating, directly imaged planet host star, Îș Andromedae
The Ages of A-Stars I: Interferometric Observations and Age Estimates for Stars in the Ursa Major Moving Group
We have observed and spatially resolved a set of seven A-type stars in the
nearby Ursa Major moving group with the Classic, CLIMB, and PAVO beam combiners
on the CHARA Array. At least four of these stars have large rotational
velocities ( 170 ) and are expected to
be oblate. These interferometric measurements, the stars' observed photometric
energy distributions, and values are used to computationally
construct model oblate stars from which stellar properties (inclination,
rotational velocity, and the radius and effective temperature as a function of
latitude, etc.) are determined. The results are compared with MESA stellar
evolution models (Paxton et al. 2011, 2013) to determine masses and ages. The
value of this new technique is that it enables the estimation of the
fundamental properties of rapidly rotating stars without the need to fully
image the star. It can thus be applied to stars with sizes comparable to the
interferometric resolution limit as opposed to those that are several times
larger than the limit. Under the assumption of coevality, the spread in ages
can be used as a test of both the prescription presented here and the MESA
evolutionary code for rapidly rotating stars. With our validated technique, we
combine these age estimates and determine the age of the moving group to be 414
23 Myr, which is consistent with, but much more precise than previous
estimates.Comment: Accepted by Ap
Unusually Wide Binaries: Are They Wide or Unusual?
We describe an astrometric and spectroscopic campaign to confirm the youth
and association of a complete sample of candidate wide companions in Taurus and
Upper Sco. Our survey found fifteen new binary systems (3 in Taurus and 12 in
Upper Sco) with separations of 3-30" (500-5000 AU) among all of the known
members with masses of 2.5-0.012 Msun. The total sample of 49 wide systems in
these two regions conforms to only some expectations from field multiplicity
surveys. Higher-mass stars have a higher frequency of wide binary companions,
and there is a marked paucity of wide binary systems near the substellar
regime. However, the separation distribution appears to be log-flat, rather
than declining as in the field, and the mass ratio distribution is more biased
toward similar-mass companions than the IMF or the field G dwarf distribution.
The maximum separation also shows no evidence of a limit at <5000 AU until the
abrupt cessation of any wide binary formation at system masses of ~0.3 Msun. We
attribute this result to the post-natal dynamical sculpting that occurs for
most field systems; our binary systems will escape to the field intact, but
most field stars are formed in denser clusters and do not. In summary, only
wide binary systems with total masses <0.3 Msun appear to be "unusually wide".Comment: Accepted to ApJ; 23 pages, 9 figures, 9 tables in emulateapj. Table 6
is online-only and is attached as tab6.te
Mapping the Shores of the Brown Dwarf Desert II: Multiple Star Formation in Taurus-Auriga
We have conducted a high-resolution imaging study of the Taurus-Auriga
star-forming region in order to characterize the primordial outcome of multiple
star formation and the extent of the brown dwarf desert. Our survey identified
16 new binary companions to primary stars with masses of 0.25-2.5 Msun, raising
the total number of binary pairs (including components of high-order multiples)
with separations of 3--5000 AU to 90. We find that ~2/3--3/4 of all Taurus
members are multiple systems of two or more stars, while the other ~1/4--1/3
appear to have formed as single stars; the distribution of high-order
multiplicity suggests that fragmentation into a wide binary has no impact on
the subsequent probability that either component will fragment again. The
separation distribution for solar-type stars (0.7--2.5 Msun) is nearly log-flat
over separations of 3--5000 AU, but lower-mass stars (0.25--0.7 Msun) show a
paucity of binary companions with separations of >200 AU. Across this full mass
range, companion masses are well described with a linear-flat function; all
system mass ratios (q=M_B/M_A) are equally probable, apparently including
substellar companions. Our results are broadly consistent with the two expected
modes of binary formation (freefall fragmentation on large scales and disk
fragmentation on small scales), but the distributions provide some clues as to
the epochs at which the companions are likely to form.Comment: Accepted to ApJ; 32 pages, 7 figures, 6 tables in emulateapj forma
Spectroscopy across the brown dwarf/planetary mass boundary - I. Near-infrared JHK spectra
With a uniform VLT SINFONI data set of nine targets, we have developed an
empirical grid of J,H,K spectra of the atmospheres of objects estimated to have
very low substellar masses of \sim5-20 MJup and young ages of \sim1-50 Myr.
Most of the targets are companions, objects which are especially valuable for
comparison with atmosphere and evolutionary models, as they present rare cases
in which the age is accurately known from the primary. Based on the sample
youth, all objects are expected to have low surface gravity, and this study
investigates the critical early phases of the evolution of substellar objects.
The spectra are compared with grids of five different theoretical atmosphere
models. This analysis represents the first systematic model comparison with
infrared spectra of young brown dwarfs. The fits to the full JHK spectra of
each object result in a range of best fit effective temperatures of +/-150-300K
whether or not the full model grid or a subset restricted to lower log(g)
values is used. This effective temperature range is significantly larger than
the uncertainty typically assigned when using a single model grid. Fits to a
single wavelength band can vary by up to 1000K using the different models.
Since the overall shape of these spectra is governed more by the temperature
than surface gravity, unconstrained model fits did not find matches with low
surface gravity or a trend in log(g) with age. This suggests that empirical
comparison with spectra of unambiguously young objects targets (such as these
SINFONI data) may be the most reliable method to search for indications of low
surface gravity and youth. For two targets, the SINFONI data are a second epoch
and the data show no variations in morphology over time. The analysis of two
other targets, AB Pic B and CT Cha B, suggests that these objects may have
lower temperatures, and consequently lower masses, than previously estimated.Comment: 15 pages, 13 figure
The ages of a-stars. I. Interferometric observations and age estimates for stars in the ursa major moving group
We have observed and spatially resolved a set of seven A-type stars in the nearby Ursa Major moving group with the Classic, CLIMB, and PAVO beam combiners on the Center for High Angular Resolution Astronomy Array. At least four of these stars have large rotational velocities (v sin i Ⳡ170 km s-1) and are expected to be oblate. These interferometric measurements, the stars observed photometric energy distributions, and v sin i values are used to computationally construct model oblate stars from which stellar properties (inclination, rotational velocity, and the radius and effective temperature as a function of latitude, etc.) are determined. The results are compared with MESA stellar evolution models to determine masses and ages. The value of this new technique is that it enables the estimation of the fundamental properties of rapidly rotating stars without the need to fully image the star. It can thus be applied to stars with sizes comparable to the interferometric resolution limit as opposed to those that are several times larger than the limit. Under the assumption of coevality, the spread in ages can be used as a test of both the prescription presented here and the MESA evolutionary code for rapidly rotating stars. With our validated technique, we combine these age estimates and determine the age of the moving group to be 414 ± 23 Myr, which is consistent with, but much more precise than previous estimates
Dynamically Driven Evolution of the Interstellar Medium in M51
Massive star formation occurs in giant molecular clouds (GMCs); an understanding of the evolution of GMCs is a prerequisite to develop theories of star formation and galaxy evolution. We report the highest-fidelity observations of the grand-design spiral galaxy M51 in carbon monoxide (CO) emission, revealing the evolution of GMCs vis-a-vis the large-scale galactic structure and dynamics. The most massive GMCs (giant molecular associations (GMAs)) are first assembled and then broken up as the gas flow through the spiral arms. The GMAs and their H_2 molecules are not fully dissociated into atomic gas as predicted in stellar feedback scenarios, but are fragmented into smaller GMCs upon leaving the spiral arms. The remnants of GMAs are detected as the chains of GMCs that emerge from the spiral arms into interarm regions. The kinematic shear within the spiral arms is sufficient to unbind the GMAs against self-gravity. We conclude that the evolution of GMCs is driven by large-scale galactic dynamicsâtheir coagulation into GMAs is due to spiral arm streaming motions upon entering the arms, followed by fragmentation due to shear as they leave the arms on the downstream side. In M51, the majority of the gas remains molecular from arm entry through the interarm region and into the next spiral arm passage
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