49 research outputs found
A Mid-Infrared Imaging Survey of Embedded Young Stellar Objects in the Rho Ophiuchi Cloud Core
Results of a comprehensive, new, ground-based mid-infrared imaging survey of
the young stellar population of the Rho Ophiuchi cloud are presented. Data were
acquired at the Palomar 5-m and at the Keck 10-m telescopes with the MIRLIN and
LWS instruments, at 0.25 arcsec and 0.25 arcsec resolutions, respectively. Of
172 survey objects, 85 were detected. Among the 22 multiple systems observed,
15 were resolved and their individual component fluxes determined. A plot of
the frequency distribution of the detected objects with SED spectral slope
shows that YSOs spend ~400,000 yr in the Flat Spectrum phase, clearing out
their remnant infall envelopes. Mid-infrared variability is found among a
significant fraction of the surveyed objects, and is found to occur for all SED
classes with optically thick disks. Large-amplitude near-infrared variability,
also found for all SED classes with optically thick disks, seems to occur with
somewhat higher frequency at the earlier evolutionary stages. Although a
general trend of mid-infrared excess and NIR veiling exists proceeding through
SED classes, with Class I objects generally exhibiting K-veilings > 1, Flat
Spectrum objects with K-veilings > 0.58, and Class III objects with K-veilings
=0, Class II objects exhibit the widest range of K-band veiling values, 0-4.5.
However, the highly variable value of veiling that a single source can exhibit
in any of the SED classes in which active disk accretion can take place is
striking, and is direct observational evidence for highly time-variable
accretion activity in disks. Finally, by comparing mid-infrared vs.
near-infrared excesses in a subsample with well-determined effective
temperatures and extinction values, disk clearing mechanisms are explored. The
results are consistent with disk clearing proceeding from the inside-out.Comment: 18 pages + 5 tables + 7 figure
Low-Mass Star Formation and the Initial Mass Function in the Rho Ophiuchi Cloud Core
We have obtained moderate-resolution (R=800-1200) K-band spectra for ~100
stars within and surrounding the cloud core of rho Oph. We have measured
spectral types and continuum veilings and have combined this information with
results from new deep imaging. The IMF peaks at about 0.4 M_sun and slowly
declines to the hydrogen burning limit with a slope of ~-0.5 in logarithmic
units (Salpeter is +1.35). Our lower limits on the numbers of substellar
objects demonstrate that the IMF probably does not fall more steeply below the
hydrogen burning limit, at least down to ~0.02 M_sun. We then make the first
comparison of mass functions of stars and pre-stellar clumps (Motte, Andre, &
Neri) measured in the same region. The similar behavior of the two mass
functions in rho Oph supports the suggestion of Motte et al. and Testi &
Sargent that the stellar mass function in young clusters is a direct product of
the process of cloud fragmentation. After considering the effect of extinction
on the SED classifications of the sample, we find that ~17% of the rho Oph
stars are Class I, implying ~0.1 Myr for the lifetime of this stage. In spectra
separated by two years, we observe simultaneous variability in the Br gamma
emission and K-band continuum veiling for two stars, where the hydrogen
emission is brighter in the more heavily veiled data. This behavior indicates
that the disk may contribute significantly to continuous K-band emission, in
contrast to the proposal that the infalling envelope always dominates. Our
detection of strong 2 micron veiling (r_K=1-4) in several Class II and III
stars, which should have disks but little envelope material, further supports
this proposition.Comment: 35 pages, 14 figures, accepted to Ap
Optical Spectroscopy of the Surface Population of the rho Ophiuchi Molecular Cloud: The First Wave of Star Formation
We present the results of optical spectroscopy of 139 stars obtained with the
Hydra multi-object spectrograph. The objects extend over a 1.3 square degree
area surrounding the main cloud of the rho Oph complex. The objects were
selected from narrowband images to have H alpha in emission. Using the presence
of strong H alpha emission, lithium absorption, location in the
Hertzsprung-Russell diagram, or previously reported x-ray emission, we were
able to identify 88 objects as young stars associated with the cloud. Strong H
alpha emission was confirmed in 39 objects with line widths consistent with
their origin in magnetospheric accretion columns. Two of the strongest
emission-line objects are young, x-ray emitting brown dwarf candidates with M8
spectral types. Comparisons of the bolometric luminosities and effective
temperatures with theoretical models suggest a medianage for this population of
2.1 Myr which is signifcantly older than the ages derived for objects in the
cloud core. It appears that these stars formed contemporaneously with low mass
stars in the Upper Scorpius subgroup, likely triggered by massive stars in the
Upper-Centaurus subgroup.Comment: 35 pages of postscript which includes seven figures (some of which
are multi-panel) and four postscript tables. Astronomical Journal (in press
A Search for Close Binaries in the Rho Ophiuchus Star-Forming Region
We have carried out a new, near-infrared speckle imaging survey of 19 members
of the young stellar population in the nearby (d=140 pc), Rho Ophiuchi cloud
core. Results for four binary and one newly discovered triple system are
reported. Data for all known multiple systems among the pre-main-sequence
population of Rho Oph are tabulated. We define a "restricted binary fraction",
Fbr, and a "restricted companion fraction", Fcr, as counting only those systems
most detectable in the present and previous high-resolution near-infrared
imaging surveys, having separations between 0.1-1.1 arcsec and K-band magnitude
differences, Delta K < 3. Analysis of all the available multiplicity data
results in updated values of Fbr=Fcr=24%+/-11% for the Ophiuchus
pre-main-sequence population. These values are consistent with the values in
the Taurus star-forming region, and Fcr is in excess by a factor of 2 relative
to the Main Sequence at the 1-sigma level.Comment: 21 pages, 6 figures, 4 table
The way of partnership
All around the globe, citizens of higher education embarked on a quest. They did so on the promise of a better future: one where they became fuller versions of themselves, empowered to be authentic, confident, passionate, articulate individuals in meaningful, trusting relationships with each other. And yet the terrain they had to cross was swampy—an uncertain, confusing, risky place that required courage and collaboration to stay their paths. The teachers had to shed their need for authority and control; the students needed to find their voice and agency. It was terrain they could not cross alone but only in the company of the other—for in sharing and listening and being there was the promise of understanding. Not all made it, for some the promise failed; but those that did saw differently and would never be the same again. They came to realise that partnership was a way of being, permeating our souls and changing the ways we learn and teach and relate; a way that lets us fully be
Circumstellar Structure around Evolved Stars in the Cygnus-X Star Formation Region
We present observations of newly discovered 24 micron circumstellar
structures detected with the Multiband Imaging Photometer for Spitzer (MIPS)
around three evolved stars in the Cygnus-X star forming region. One of the
objects, BD+43 3710, has a bipolar nebula, possibly due to an outflow or a
torus of material. A second, HBHA 4202-22, a Wolf-Rayet candidate, shows a
circular shell of 24 micron emission suggestive of either a limb-brightened
shell or disk seen face-on. No diffuse emission was detected around either of
these two objects in the Spitzer 3.6-8 micron Infrared Array Camera (IRAC)
bands. The third object is the luminous blue variable candidate G79.29+0.46. We
resolved the previously known inner ring in all four IRAC bands. The 24 micron
emission from the inner ring extends ~1.2 arcmin beyond the shorter wavelength
emission, well beyond what can be attributed to the difference in resolutions
between MIPS and IRAC. Additionally, we have discovered an outer ring of 24
micron emission, possibly due to an earlier episode of mass loss. For the two
shell stars, we present the results of radiative transfer models, constraining
the stellar and dust shell parameters. The shells are composed of amorphous
carbon grains, plus polycyclic aromatic hydrocarbons in the case of
G79.29+0.46. Both G79.29+0.46 and HBHA 4202-22 lie behind the main Cygnus-X
cloud. Although G79.29+0.46 may simply be on the far side of the cloud, HBHA
4202-22 is unrelated to the Cygnus-X star formation region.Comment: Accepted by A
The deuteron: structure and form factors
A brief review of the history of the discovery of the deuteron in provided.
The current status of both experiment and theory for the elastic electron
scattering is then presented.Comment: 80 pages, 33 figures, submited to Advances in Nuclear Physic
The outskirts of Cygnus OB2
We investigate the possible existence of an extended halo of early-type stars
around Cygnus OB2, which is hinted at by near-infrared color-color diagrams,
and its relationship to Cygnus OB2 itself, as well as to the nearby association
Cygnus OB9 and to the star forming regions in the Cygnus X North complex. A
total of 96 early-type stars are identified in the targeted region, which
amounts to nearly half of the observed sample. Most of them have featureless
near-infrared spectra as expected from OB stars at the available resolution.
Another 18 stars that display Brackett emission lines can be divided between
evolved massive stars (most likely Be stars) and Herbig Ae/Be stars based on
their infrared excesses. A component associated with Cygnus OB9/NGC 6910 is
clearly identified, as well as an enhancement in the surface density of
early-type stars at Cygnus X North. We also find a field population, consisting
largely of early B giants and supergiants, which is probably the same as
identified in recent studies of the inner 1-degree circle around Cygnus OB2.
The age and large extension of this population discards a direct relationship
with Cygnus OB2 or any other particular association. Earlier claims of the
possible large extent of Cygnus OB2 beyond its central, very massive aggregate
seem to be dismissed by our findings. The existence of a nearly ubiquitous
population of evolved stars with massive precursors suggests a massive star
formation history in Cygnus having started long before the formation of the
currently observed OB associations in the region.Comment: 22 pages, 12 figures. Accepted by Astronomy and Astrophysic
Can environment or allergy explain international variation in prevalence of wheeze in childhood?
Asthma prevalence in children varies substantially around the world, but the contribution of known risk factors to this international variation is uncertain. The International Study of Asthma and Allergies in Childhood (ISAAC) Phase Two studied 8–12 year old children in 30 centres worldwide with parent-completed symptom and risk factor questionnaires and aeroallergen skin prick testing. We used multilevel logistic regression modelling to investigate the effect of adjustment for individual and ecological risk factors on the between-centre variation in prevalence of recent wheeze. Adjustment for single individual-level risk factors changed the centre-level variation from a reduction of up to 8.4% (and 8.5% for atopy) to an increase of up to 6.8%. Modelling the 11 most influential environmental factors among all children simultaneously, the centre-level variation changed little overall (2.4% increase). Modelling only factors that decreased the variance, the 6 most influential factors (synthetic and feather quilt, mother’s smoking, heating stoves, dampness and foam pillows) in combination resulted in a 21% reduction in variance. Ecological (centre-level) risk factors generally explained higher proportions of the variation than did individual risk factors. Single environmental factors and aeroallergen sensitisation measured at the individual (child) level did not explain much of the between-centre variation in wheeze prevalence