548 research outputs found
A new mid-infrared map of the BN/KL region using the Keck telescope
We present a new mid-infrared (12.5micron) map of the BN/KL high-mass
star-forming complex in Orion using the LWS instrument at Keck I. Despite poor
weather we achieved nearly diffraction-limited images (FWHM = 0.38'') over a
roughly 25'' X 25'' region centered on IRc2 down to a flux limit of ~250 mJy.
Many of the known infrared (IR) sources in the region break up into smaller
sub-components. We have also detected 6 new mid-IR sources. Nearly all of the
sources are resolved in our mosaic. The near-IR source ''n'' is slightly
elongated in the mid-IR along a NW--SE axis and perfectly bisects the
double-peaked radio source ''L''. Source n has been identified as a candidate
for powering the large IR luminosity of the BN/KL region (L = 10^5 L_sun). We
postulate that the 12 micron emission arises in a circumstellar disk
surrounding source n. The morphology of the mid-IR emission and the Orion ''hot
core'' (as seen in NH_3 emission), along with the location of water and OH
masers, is very suggestive of a bipolar cavity centered on source n and aligned
with the rotation axis of the hypothetical circumstellar disk. IRc2, once
thought to be the dominant energy source for the BN/KL region, clearly breaks
into 4 sub-sources in our mosaic, as seen previously at 3.8 -- 5.0 micron. The
anti-correlation of mid-IR emission and NH_3 emission from the nearby hot core
indicates that the IRc2 sources are roughly coincident (or behind) the dense
hot core. The nature of IRc2 is not clear: neither self-luminous sources
(embedded protostars) nor external heating by source I can be definitively
ruled out. We also report the discovery of a new arc-like feature SW of the BN
object, and some curious morphology surrounding near-IR source ''t".Comment: To appear in The Astronomical Journal, July 2004 (16 pages, 7
figures
Magnetic Fields in Star-Forming Molecular Clouds I. The First Polarimetry of OMC-3 in Orion A
The first polarimetric images of the OMC-3 region of the Orion A filamentary
molecular cloud are presented. Using the JCMT, we have detected polarized
thermal emission at 850 microns from dust along a 6' length of the dense
filament. The polarization pattern is highly ordered and is aligned with the
filament throughout most of the region. The plane-of-sky magnetic field
direction is perpendicular to the measured polarization. The mean percentage
polarization is 4.2% with a 1 sigma dispersion of 1%. This region is part of
the integral-shaped filament, and active star formation is ongoing along its
length. The protostellar outflow directions do not appear to be consistently
correlated with the direction of the plane-of-sky field or the filament
structure itself. Depolarization toward the filament center, previously
detected in many other star-forming cores and protostars, is also evident in
our data. (abstract abridged)Comment: 9 pages plus 2 figures (1 colour); accepted for publication in the
March 10, 2000 issue (vol. 531 #2) of The Astrophysical Journa
"GiGa": the Billion Galaxy HI Survey -- Tracing Galaxy Assembly from Reionization to the Present
In this paper, we review the Billion Galaxy Survey that will be carried out
at radio--optical wavelengths to micro--nanoJansky levels with the telescopes
of the next decades. These are the Low-Frequency Array, the Square Kilometer
Array and the Large Synoptic Survey Telescope as survey telescopes, and the
Thirty Meter class Telescopes for high spectral resolution+AO, and the James
Webb Space Telescope (JWST) for high spatial resolution near--mid IR follow-up.
With these facilities, we will be addressing fundamental questions like how
galaxies assemble with super-massive black-holes inside from the epoch of First
Light until the present, how these objects started and finished the
reionization of the universe, and how the processes of star-formation, stellar
evolution, and metal enrichment of the IGM proceeded over cosmic time. We also
summarize the high-resolution science that has been done thus far on high
redshift galaxies with the Hubble Space Telescope (HST). Faint galaxies have
steadily decreasing sizes at fainter fluxes and higher redshifts, reflecting
the hierarchical formation of galaxies over cosmic time. HST has imaged this
process in great structural detail to z<~6. We show that ultradeep
radio-optical surveys may slowly approach the natural confusion limit, where
objects start to unavoidably overlap because of their own sizes, which only SKA
can remedy with HI redshifts for individual sub-clumps. Finally, we summarize
how the 6.5 meter James Webb Space Telescope (JWST) will measure first light,
reionization, and galaxy assembly in the near--mid-IR.Comment: 8 pages, LaTeX2e requires 'aip' style (included), 8 postscript
figures. To appear in the proceedings of the `The Evolution of Galaxies
through the Neutral Hydrogen Window' conference, Arecibo Observatory Feb 1-3,
2008; Eds. R. Minchin & E. Momjian, AIP Conf Pro
An equatorial wind from the massive young stellar object S140 IRS 1
The discovery of the second equatorial ionized stellar wind from a massive
young stellar object is reported. High resolution radio continuum maps of S140
IRS 1 reveal a highly elongated source that is perpendicular to the larger
scale bipolar molecular outflow. This picture is confirmed by location of a
small scale monopolar near-IR reflection nebula at the base of the blueshifted
lobe. A second epoch of observations over a five year baseline show little
ordered outward proper motion of clumps as would have been expected for a jet.
A third epoch, taken only 50 days after the second, did show significant
changes in the radio morphology. These radio properties can all be understood
in the context of an equatorial wind driven by radiation pressure from the
central star and inner disc acting on the gas in the surface layers of the disc
as proposed by Drew et al. (1998). This equatorial wind system is briefly
compared with the one in S106IR, and contrasted with other massive young
stellar objects that drive ionized jets.Comment: 19 pages, 5 figures, accepted by ApJ, minor changes in light of
referees repor
The escallop, Pecten maximus (L.), in Killary Harbour
Escallops (843) collected by SCUBA diving ranged from one year to an estimated seventeen years old and varied in shell height from 17mm to 164mm. They were predominantly found on the slopes on either side of the Harbour at depths between 6m and 35m; very few specimens were observed on the soft mud in the centre of the Harbour. The density of escallops increased towards the mouth of the Harbour.
The greatest numbers of specimens of less than 50mm were observed at the mouth of Killary. Variability in age frequency distributions along either side of the harbour could be explained by differences in settlement intensity from year to year. Settlement appeared to be more successful on the southern side than on the northern side. It also appeared that local dispersion from settlement areas occurred rather than active migration. Escallops four years old or less swam regularly while swimming was less frequently seen in individuals exceeding five years of age.
It was concluded that Killary Harbour was unsuitable for the collection of spat but may be a suitable area for escallop cultivation
Disks around massive young stellar objects: are they common?
We present K-band polarimetric images of several massive young stellar
objects at resolutions 0.1-0.5 arcsec. The polarization vectors around
these sources are nearly centro-symmetric, indicating they are dominating the
illumination of each field. Three out of the four sources show elongated
low-polarization structures passing through the centers, suggesting the
presence of polarization disks. These structures and their surrounding
reflection nebulae make up bipolar outflow/disk systems, supporting the
collapse/accretion scenario as their low-mass siblings. In particular, S140
IRS1 show well defined outflow cavity walls and a polarization disk which
matches the direction of previously observed equatorial disk wind, thus
confirming the polarization disk is actually the circumstellar disk. To date, a
dozen massive protostellar objects show evidence for the existence of disks;
our work add additional samples around MYSOs equivalent to early B-type stars.Comment: 9 pages, including 2 figures, 1 table, to appear on ApJ
Chandra Study of the Cepheus B Star Forming Region: Stellar Populations and the Initial Mass Function
Cepheus B (Cep B) molecular cloud and a portion of the nearby Cep OB3b OB
association, one of the most active regions of star formation within 1 kpc, has
been observed with the ACIS detector on board the Chandra X-ray Observatory. We
detect 431 X-ray sources, of which 89% are confidently identified as clustered
pre-main sequence stars. Two main results are obtained. First, we provide the
best census to date for the stellar population of the region. We identify many
members of two rich stellar clusters: the lightly obscured Cep OB3b
association, and the deeply embedded cluster in Cep B whose existence was
previously traced only by a handful of radio sources and T Tauri stars. Second,
we find a discrepancy between the X-ray Luminosity Functions of the Cep OB3b
and the Orion Nebula Cluster. This may be due to different Initial Mass
Functions of two regions (excess of ~0.3 solar mass stars), or different age
distributions. Several other results are obtained. A diffuse X-ray component
seen in the field is attributed to the integrated emission of unresolved low
mass PMS stars. The X-ray emission from HD 217086 (O7n), the principle ionizing
source of the region, follows the standard model involving many small shocks in
an unmagnetized radiatively accelerated wind. The X-ray source #294 joins a
number of similar superflare PMS stars where long magnetic structures may
connect the protoplanetary disk to the stellar surface.Comment: 72 pages, 31 figures, 8 tables. Accepted for publication in Ap
Multiple Sources toward the High-mass Young Star S140 IRS1
S140 IRS1 is a remarkable source where the radio source at the center of the
main bipolar molecular outflow in the region is elongated perpendicular to the
axis of the outflow, an orientation opposite to that expected if the radio
source is a thermal jet exciting the outflow. We present results of 1.3 cm
continuum and H2O maser emission observations made with the VLA in its A
configuration toward this region. In addition, we also present results of
continuum observations at 7 mm and re-analyse observations at 2, 3.5 and 6 cm
(previously published). IRS 1A is detected at all wavelengths, showing an
elongated structure. Three water maser spots are detected along the major axis
of the radio source IRS 1A. We have also detected a new continuum source at 3.5
cm (IRS 1C) located ~0.6'' northeast of IRS 1A. The presence of these two YSOs
(IRS 1A and 1C) could explain the existence of the two bipolar molecular
outflows observed in the region. In addition, we have also detected three
continuum clumps (IRS 1B, 1D and 1E) located along the major axis of IRS 1A. We
discuss two possible models to explain the nature of IRS 1A: a thermal jet and
an equatorial wind.Comment: 17 pages, 4 figures, to be published in A
Equation of State for Neutralino Star as a Form of Cold Dark Matter
In order to study the structure of neutralino star and dark galaxy, we
consider dynamical interactions due to boson-exchange in the neutralino matter.
Taking into account interactions of neutralinos with bosons, we derive the
equation of state (EOS) of neutralino stars in terms of the relativistic mean
field approach. Then we apply the resulting EOS to investigate properties of
the neutralino star such as its density profile and mass limit. For example, if
the neutralino mass is around 1 TeV, the Oppenheimer mass limit of the
neutralino star is obtained as , and the
corresponding radius is about 7.8 mm. Actually, due to an increasing
annihilation rate as indicated by our calculation, this dense state can never
be realized in practice. Our results also show that the low density neutralino
star may be a possible aggregation of the cold dark matter.Comment: 5 pages, 5 figures; v2: matches published versio
Mid-infrared imaging of the massive young star AFGL 2591: Probing the circumstellar environment of an outflow source
Most, if not all, stars are now believed to produce energetic outflows during
their formation. Yet, almost 20 years after the discovery of bipolar outflows
from young stars, the origins of this violent phenomenon are not well
understood. One of the difficulties of probing the outflow process,
particularly in the case of massive embedded stars, is a deficit of high
spatial resolution observations. Here, we present sub-arcsecond-resolution
mid-infrared images of one massive young stellar object, AFGL 2591, and its
immediate surroundings. Our images, at 11.7, 12.5 and 18.0 microns, reveal a
knot of emission ~6'' SW of the star, which may be evidence for a recent
ejection event or an embedded companion star. This knot is roughly coincident
with a previously seen near-infrared reflection nebula and a radio source, and
lies within the known large-scale CO outflow. We also find a new faint NW
source which may be another embedded lower-luminosity star. The IRAS
mid-infrared spectrum of AFGL 2591 shows a large silicate absorption feature at
10 microns, implying that the primary source is surrounded by an optically
thick dusty envelope. We discuss the interrelationship of these phenomena and
suggest that mid-infrared imaging and spectroscopy provide powerful tools for
probing massive star birth.Comment: 14 pages, 3 PostScript figures, accepted for publication in The
Astrophysical Journal Letter
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