63 research outputs found
Testing for Photometric Variability at the L/T Boundary
We construct K_s-band light curves for nine field L and T
brown dwarfs using the Palomar 60 inch Telescope. Results of a robust
statistical analysis indicate that about half the targets show significant
evidence for variability. Two of these variable targets have marginally
significant peaks in the Lomb-Scargle periodogram. The phased light
curves show evidence for periodic behavior on timescales of about 1.5
and 3.0 hours
Disk and Envelope Structure in Class 0 Protostars: I. The Resolved Massive Disk in Serpens FIRS 1
We present the first results of a program to characterize the disk and
envelope structure of typical Class 0 protostars in nearby low-mass star
forming regions. We use Spitzer IRS mid-infrared spectra, high resolution CARMA
230 GHz continuum imaging, and 2-D radiative transfer models to constrain the
envelope structure, as well as the size and mass of the circum-protostellar
disk in Serpens FIRS 1. The primary envelope parameters (centrifugal radius,
outer radius, outflow opening angle, and inclination) are well constrained by
the spectral energy distribution (SED), including Spitzer IRAC and MIPS
photometry, IRS spectra, and 1.1 mm Bolocam photometry. These together with the
excellent uv-coverage (4.5-500 klam) of multiple antenna configurations with
CARMA allow for a robust separation of the envelope and a resolved disk. The
SED of Serpens FIRS 1 is best fit by an envelope with the density profile of a
rotating, collapsing spheroid with an inner (centrifugal) radius of
approximately 600 AU, and the millimeter data by a large resolved disk with
Mdisk~1.0 Msun and Rdisk~300 AU. These results suggest that large, massive
disks can be present early in the main accretion phase. Results for the larger,
unbiased sample of Class~0 sources in the Perseus, Serpens, and Ophiuchus
molecular clouds are needed to determine if relatively massive disks are
typical in the Class 0 stage.Comment: Comments: 13 pages, 8 figures, 3 tables; accepted for publication in
the Ap
Properties of the Youngest Protostars in Perseus, Serpens, and Ophiuchus
We present an unbiased census of deeply embedded protostars in Perseus,
Serpens, and Ophiuchus, assembled by combining large-scale 1.1 mm Bolocam
continuum and Spitzer Legacy surveys. We identify protostellar candidates based
on their mid-infrared properties, correlate their positions with 1.1 mm core
positions, and construct well-sampled SEDs using our extensive wavelength
coverage (lam=1.25-1100 micron). Source classification based on the bolometric
temperature yields a total of 39 Class 0 and 89 Class I sources in the three
cloud sample. We compare to protostellar evolutionary models using the
bolometric temperature-luminosity diagram, finding a population of low
luminosity Class I sources that are inconsistent with constant or monotonically
decreasing mass accretion rates. This result argues strongly for episodic
accretion during the Class I phase, with more than 50% of sources in a
``sub-Shu'' (dM/dt < 1e-6 Msun/yr) accretion state. Average spectra are
compared to protostellar radiative transfer models, which match the observed
spectra fairly well in Stage 0, but predict too much near-IR and too little
mid-IR flux in Stage I. Finally, the relative number of Class 0 and Class I
sources are used to estimate the lifetime of the Class 0 phase; the three cloud
average yields a Class 0 lifetime of 1.7e5 yr, ruling out an extremely rapid
early accretion phase. Correcting photometry for extinction results in a
somewhat shorter lifetime (1.1e5 yr). In Ophiuchus, however, we find very few
Class 0 sources (N(Class0)/N(ClassI)=0.1-0.2), similar to previous studies of
that cloud. The observations suggest a consistent picture of nearly constant
average accretion rate through the entire embedded phase, with accretion
becoming episodic by at least the Class I stage, and possibly earlier.Comment: 31 pages, 19 figures, 8 tables; accepted for publication in Ap
Comparing Star Formation on Large Scales in the c2d Legacy Clouds: Bolocam 1.1 mm Dust Continuum Surveys of Serpens, Perseus, and Ophiuchus
We have undertaken an unprecedentedly large 1.1 millimeter continuum survey
of three nearby star forming clouds using Bolocam at the Caltech Submillimeter
Observatory. We mapped the largest areas in each cloud at millimeter or
submillimeter wavelengths to date: 7.5 sq. deg in Perseus (Paper I), 10.8 sq.
deg in Ophiuchus (Paper II), and 1.5 sq. deg in Serpens with a resolution of
31", detecting 122, 44, and 35 cores, respectively. Here we report on results
of the Serpens survey and compare the three clouds. Average measured angular
core sizes and their dependence on resolution suggest that many of the observed
sources are consistent with power-law density profiles. Tests of the effects of
cloud distance reveal that linear resolution strongly affects measured source
sizes and densities, but not the shape of the mass distribution. Core mass
distribution slopes in Perseus and Ophiuchus (alpha=2.1+/-0.1 and
alpha=2.1+/-0.3) are consistent with recent measurements of the stellar IMF,
whereas the Serpens distribution is flatter (alpha=1.6+/-0.2). We also compare
the relative mass distribution shapes to predictions from turbulent
fragmentation simulations. Dense cores constitute less than 10% of the total
cloud mass in all three clouds, consistent with other measurements of low
star-formation efficiencies. Furthermore, most cores are found at high column
densities; more than 75% of 1.1 mm cores are associated with Av>8 mag in
Perseus, 15 mag in Serpens, and 20-23 mag in Ophiuchus.Comment: 32 pages, including 18 figures, accepted for publication in Ap
A complete 1.1mm survey of Perseus with Bolocam
We have completed a 7.5 square degree λ=1.1mm map of Perseus
using Bolocam. Our map is the largest unbiased survey of Perseus at millimeter wavelengths to date, and covers the same area as the c2d Spitzer IRAC and MIPS maps of Perseus. We find that that the mass function shape is similar to that seen in other clouds and to the local IMF. Despite the large area surveyed, few new sources are found outside the known cluster regions
Bolocam Survey for 1.1 mm Dust Continuum Emission in the c2d Legacy Clouds. II. Ophiuchus
We present a large-scale millimeter continuum map of the Ophiuchus molecular
cloud. Nearly 11 square degrees, including all of the area in the cloud with
visual extinction more than 3 magnitudes, was mapped at 1.1 mm with Bolocam on
the Caltech Submillimeter Observatory (CSO). By design, the map also covers the
region mapped in the infrared with the Spitzer Space Telescope. We detect 44
definite sources, and a few likely sources are also seen along a filament in
the eastern streamer. The map indicates that dense cores in Ophiuchus are very
clustered and often found in filaments within the cloud. Most sources are
round, as measured at the half power point, but elongated when measured at
lower contour levels, suggesting spherical sources lying within filaments. The
masses, for an assumed dust temperature of 10 K, range from 0.24 to 3.9 solar
masses, with a mean value of 0.96 solar masses. The total mass in distinct
cores is 42 solar masses, 0.5 to 2% of the total cloud mass, and the total mass
above 4 sigma is about 80 solar masses. The mean densities in the cores are
quite high, with an average of 1.6 x 10^6 per cc, suggesting short free-fall
times. The core mass distribution can be fitted with a power law with slope of
2.1 plus or minus 0.3 for M>0.5 solar masses, similar to that found in other
regions, but slightly shallower than that of some determinations of the local
IMF. In agreement with previous studies, our survey shows that dense cores
account for a very small fraction of the cloud volume and total mass. They are
nearly all confined to regions with visual extinction at least 9 mag, a lower
threshold than found previously.Comment: 47 pages, 16 figures, accepted for Ap
Photometric Variability at the L/T Dwarf Boundary
We have monitored the photometric variability of nine field L and T brown
dwarfs for 10 nights during the course of one month with the Palomar 60 inch
Telescope Near-Infrared Camera. Results of statistical analyses indicate that
at least three of the nine targets show significant evidence for variability,
and three more are possibly variable. Fractional deviations from the median
flux vary from 5% to 25%. Two of the variable targets, 2MASS 0030-14 (L7) and
SDSS 0151+12 (T1), have marginally significant peaks in their periodograms. The
phased light curves show evidence for periodic behavior on timescales of 1.5
hours and 3.0 hours respectively. No significant correlations between
variability amplitude and spectral type or (J-K_s) color are found. While it is
clear that variability exists in objects near the L/T dwarf boundary, we find
no evidence that variability near the L/T boundary is more likely than it is
for early L dwarfs.Comment: 29 pages including 11 figures, 4 tables; accepted for publication in
A
Detection of a Bipolar Molecular Outflow Driven by a Candidate First Hydrostatic Core
We present new 230 GHz Submillimeter Array observations of the candidate
first hydrostatic core Per-Bolo 58. We report the detection of a 1.3 mm
continuum source and a bipolar molecular outflow, both centered on the position
of the candidate first hydrostatic core. The continuum detection has a total
flux density of 26.6 +/- 4.0 mJy, from which we calculate a total (gas and
dust) mass of 0.11 +/- 0.05 Msun and a mean number density of 2.0 +/- 1.6 X
10^7 cm-3. There is some evidence for the existence of an unresolved component
in the continuum detection, but longer-baseline observations are required in
order to confirm the presence of this component and determine whether its
origin lies in a circumstellar disk or in the dense inner envelope. The bipolar
molecular outflow is observed along a nearly due east-west axis. The outflow is
slow (characteristic velocity of 2.9 km/s), shows a jet-like morphology
(opening semi-angles ~8 degrees for both lobes), and extends to the edges of
the primary beam. We calculate the kinematic and dynamic properties of the
outflow in the standard manner and compare them to several other protostars and
candidate first hydrostatic cores with similarly low luminosities. We discuss
the evidence both in support of and against the possibility that Per-Bolo 58 is
a first hydrostatic core, and we outline future work needed to further evaluate
the evolutionary status of this object.Comment: 11 Pages, 6 Figures, 4 Tables. Accepted by Ap
Disk and Envelope Structure in Class 0 Protostars: II. High Resolution Millimeter Mapping of the Serpens Sample
We present high-resolution CARMA 230 GHz continuum imaging of nine deeply
embedded protostars in the Serpens Molecular Cloud, including six of the nine
known Class 0 protostars in Serpens. This work is part of a program to
characterize disk and envelope properties for a complete sample of Class 0
protostars in nearby low-mass star forming regions. Here we present CARMA maps
and visibility amplitudes as a function of uv-distance for the Serpens sample.
Observations are made in the B, C, D, and E antenna configurations, with B
configuration observations utilizing the CARMA Paired Antenna Calibration
System. Combining data from multiple configurations provides excellent
uv-coverage (4-500 klam), allowing us to trace spatial scales from 1e2 to 1e4
AU. We find evidence for compact disk components in all of the observed Class 0
protostars, suggesting that disks form at very early times (t<0.2 Myr) in
Serpens. We make a first estimate of disk masses using the flux at 50 klam,
where the contribution from the envelope should be negligible, assuming an
unresolved disk. The resulting disk masses range from 0.04 Msun to 1.7 Msun,
with a mean of approximately 0.2 Msun. Our high resolution maps are also
sensitive to binary or multiple sources with separations > 250 AU, but
significant evidence of multiplicity on scales <2000 AU is seen in only one
source.Comment: Accepted to the Astrophysical Journal Supplement
The Mass Distribution and Lifetime of Prestellar Cores in Perseus, Serpens, and Ophiuchus
We present an unbiased census of starless cores in Perseus, Serpens, and
Ophiuchus, assembled by comparing large-scale Bolocam 1.1 mm continuum emission
maps with Spitzer c2d surveys. We use the c2d catalogs to separate 108 starless
from 92 protostellar cores in the 1.1 mm core samples from Enoch et al. (2006),
Young et al. (2006), and Enoch et al. (2007). A comparison of these populations
reveals the initial conditions of the starless cores. Starless cores in Perseus
have similar masses but larger sizes and lower densities on average than
protostellar cores, with sizes that suggest density profiles substantially
flatter than r^-2. By contrast, starless cores in Serpens are compact and have
lower masses than protostellar cores; future star formation will likely result
in lower mass objects than the currently forming protostars. Comparison to
dynamical masses estimated from the NH_3 survey of Perseus cores by Rosolowsky
et al. (2007) suggests that most of the starless cores are likely to be
gravitationally bound, and thus prestellar. The combined prestellar core mass
distribution includes 108 cores and has a slope of -2.3+/-0.4 for M>0.8 Msun.
This slope is consistent with recent measurements of the stellar initial mass
function, providing further evidence that stellar masses are directly linked to
the core formation process. We place a lower limit on the core-to-star
efficiency of 25%. There are approximately equal numbers of prestellar and
protostellar cores in each cloud, thus the dense prestellar core lifetime must
be similar to the lifetime of embedded protostars, or 4.5x10^5 years, with a
total uncertainty of a factor of two. Such a short lifetime suggests a dynamic,
rather than quasi-static, core evolution scenario, at least at the relatively
high mean densities (n>2x10^4 cm^-3) to which we are sensitive.Comment: 27 pages, 15 figures, 5 tables, accepted for publication in ApJ.
Version with full resolution figures available at
http://www.astro.caltech.edu/~menoch/corespaper
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