4,117 research outputs found
The Magnetic Field of L1544: I. Near-Infrared Polarimetry and the Non-Uniform Envelope
The magnetic field (B-field) of the starless dark cloud L1544 has been
studied using near-infrared (NIR) background starlight polarimetry (BSP) and
archival data in order to characterize the properties of the plane-of-sky
B-field. NIR linear polarization measurements of over 1,700 stars were obtained
in the H-band and 201 of these were also measured in the K-band. The NIR BSP
properties are correlated with reddening, as traced using the RJCE (H-M)
method, and with thermal dust emission from the L1544 cloud and envelope seen
in Herschel maps. The NIR polarization position angles change at the location
of the cloud and exhibit their lowest dispersion of position angles there,
offering strong evidence that NIR polarization traces the plane-of-sky B-field
of L1544. In this paper, the uniformity of the plane-of-sky B-field in the
envelope region of L1544 is quantitatively assessed. This allowed evaluating
the approach of assuming uniform field geometry when measuring relative
mass-to-flux ratios in the cloud envelope and core based on averaging of the
envelope radio Zeeman observations, as in Crutcher et al. (2009). In L1544, the
NIR BSP shows the envelope B-field to be significantly non-uniform and likely
not suitable for averaging Zeeman properties without treating intrinsic
variations. Deeper analyses of the NIR BSP and related data sets, including
estimates of the B-field strength and testing how it varies with position and
gas density, are the subjects of later papers in this series.Comment: 16 pages, 9 figures; accepted for publication in The Astrophysical
Journa
Resolving distance ambiguities towards 6.7 GHz methanol masers
Distances to most star forming regions are determined using kinematics,
through the assumption that the observed radial velocity arises from the motion
of the source with respect to the Sun resulting from the differential rotation
of Galaxy. The primary challenge associated with the application of this
technique in the inner Galaxy is the kinematic distance ambiguity. In this
work, we aim to resolve the kinematic distance ambiguity towards a sample of
6.7 GHz methanol masers, which are signposts of the early stages of massive
star formation. We measured 21 cm HI absorption spectra using the Very Large
Array in C and CnB configurations. A comparison of the maximum velocity of HI
absorption with the source velocity and tangent point velocity was used to
resolve the kinematic distance ambiguity. We resolved the distance ambiguity
towards 41 sources. Distance determinations that are in conflict with previous
measurements are discussed. The NE2001 spiral arm model is broadly consistent
with the locations of the star forming complexes. We find that the use of
vertical scale height arguments to resolve the distance ambiguity can lead to
erroneous classifications for a significant fraction of sources.Comment: Accepted by Astronomy & Astrophysic
CO mapping of the Orion molecular cloud: The influence of star formation on cloud structure
Regions of massive star formation have long been believed to have a profound influence on the structure of their surrounding molecular clouds. The ways in which massive star formation has altered the structure and kinematics of the Orion Molecular Cloud are discussed. The data to be discussed consists of a large scale map of the CO J=1-0 emission from approximately 3 square degrees of OMC-1. During 1985, the Five College Radio Astronomy Observatory 14M antenna was used to map a 2 deg x 1 deg region centered on alpha(1950) = 5(h)33(m)00(s) delta(1950) = -5 deg 30 min. The region mapped in 1985 covers the well known HII regions M42, M43, and NGC1977, and the CO map contains abundant evidence of the interaction between these regions and the molecular cloud. Indeed, the global structure of the cloud appears to have been strongly influenced by the continuous formation of massive stars within the cloud. Individual instances of some of these features are discussed. There appear to be two classes of features which are indicative of this interaction: CO bright rims and CO holes. During 1986, we have undertaken further mapping of OMC-1 to the south of the region covered by the 1985 map. This portion of the cloud contains significant regions of star formation, but O star formation has not occured and large HII regions have not developed to alter the appearance of the cloud. A detailed map of this region is thus an opportunity to view the structure of the molecular cloud before it has been altered by massive star formation. Preliminary analysis of data obtained in this region suggests that the structure and kinematics of the southern portion of the Orion cloud are indeed dramatically different from those of the region previously mapped. Comparison of the two regions thus supports models of the development of structure in molecular clouds through interaction with the HII regions formed within them
A Herschel [C II] Galactic plane survey II: CO-dark H2 in clouds
ABRIDGED: Context: HI and CO large scale surveys of the Milky Way trace the
diffuse atomic clouds and the dense shielded regions of molecular hydrogen
clouds. However, until recently, we have not had spectrally resolved C+ surveys
to characterize the photon dominated interstellar medium, including, the H2 gas
without C, the CO-dark H2, in a large sample of clouds. Aims: To use a sparse
Galactic plane survey of the 1.9 THz [C II] spectral line from the Herschel
Open Time Key Programme, Galactic Observations of Terahertz C+ (GOT C+), to
characterize the H2 gas without CO in a statistically significant sample of
clouds. Methods: We identify individual clouds in the inner Galaxy by fitting
[CII] and CO isotopologue spectra along each line of sight. We combine these
with HI spectra, along with excitation models and cloud models of C+, to
determine the column densities and fractional mass of CO-dark H2 clouds.
Results: We identify 1804 narrow velocity [CII] interstellar cloud components
in different categories. About 840 are diffuse molecular clouds with no CO, 510
are transition clouds containing [CII] and 12CO, but no 13CO, and the remainder
are dense molecular clouds containing 13CO emission. The CO-dark H2 clouds are
concentrated between Galactic radii 3.5 to 7.5 kpc and the column density of
the CO-dark H2 layer varies significantly from cloud-to-cloud with an average
9X10^(20) cm-2. These clouds contain a significant fraction of CO-dark H2 mass,
varying from ~75% for diffuse molecular clouds to ~20% for dense molecular
clouds. Conclusions: We find a significant fraction of the warm molecular ISM
gas is invisible in HI and CO, but is detected in [CII]. The fraction of
CO-dark H2 is greatest in the diffuse clouds and decreases with increasing
total column density, and is lowest in the massive clouds.Comment: 21 pages, 19 figures, accepted for publication in A&A (2014
Striations in the Taurus molecular cloud: Kelvin-Helmholtz instability or MHD waves?
The origin of striations aligned along the local magnetic field direction in
the translucent envelope of the Taurus molecular cloud is examined with new
observations of 12CO and 13CO J=2-1 emission obtained with the 10~m
submillimeter telescope of the Arizona Radio Observatory. These data identify a
periodic pattern of excess blue and redshifted emission that is responsible for
the striations. For both 12CO and 13CO, spatial variations of the J=2-1 to
J=1-0 line ratio are small and are not spatially correlated with the striation
locations. A medium comprised of unresolved CO emitting substructures (cells)
with a beam area filling factor less than unity at any velocity is required to
explain the average line ratios and brightness temperatures. We propose that
the striations result from the modulation of velocities and the beam filling
factor of the cells as a result of either the Kelvin-Helmholtz instability or
magnetosonic waves propagating through the envelope of the Taurus molecular
cloud. Both processes are likely common features in molecular clouds that are
sub-Alfvenic and may explain low column density, cirrus-like features similarly
aligned with the magnetic field observed throughout the interstellar medium in
far-infrared surveys of dust emission.Comment: 11 pages, 4 figures. Accepted for publication in MNRA
Constraints on the Stellar/Sub-stellar Mass Function in the Inner Orion Nebula Cluster
We present the results of a 0.5-0.9" FWHM imaging survey at K (2.2 micron)
and H (1.6 micron) covering 5.1' x 5.1' centered on Theta 1C Ori, the most
massive star in the Orion Nebula Cluster (ONC). At the age and distance of this
cluster, and in the absence of extinction, the hydrogen burning limit (0.08 Mo)
occurs at K~13.5 mag while an object of mass 0.02 Mo has K~16.2 mag. Our
photometry is complete for source detection at the 7 sigma level to K~17.5 mag
and thus is sensitive to objects as low-mass as 0.02 Mo seen through visual
extinction values as high as 10 magnitudes. We use the observed magnitudes,
colors, and star counts to constrain the shape of the inner ONC stellar mass
function across the hydrogen burning limit. After determining the stellar age
and near-infrared excess properties of the optically visible stars in this same
inner ONC region, we present a new technique that incorporates these
distributions when extracting the mass function from the observed density of
stars in the K-(H-K) diagram. We find that our data are inconsistent with a
mass function that rises across the stellar/sub-stellar boundary. Instead, we
find that the most likely form of the inner ONC mass function is one that rises
to a peak around 0.15 Mo, and then declines across the hydrogen-burning limit
with slope N(log M) ~ M^(0.57+/-0.05). We emphasize that our conclusions apply
to the inner 0.71 pc x 0.71 pc of the ONC only; they may not apply to the ONC
as a whole where some evidence for general mass segregation has been found.Comment: Accepted for publication in the Astrophysical Journal.
Preprints/tables also available at http://phobos.caltech.edu/~jmc/papers/onc
Ionized gas at the edge of the Central Molecular Zone
To determine the properties of the ionized gas at the edge of the CMZ near
Sgr E we observed a small portion of the edge of the CMZ near Sgr E with
spectrally resolved [C II] 158 micron and [N II] 205 micron fine structure
lines at six positions with the GREAT instrument on SOFIA and in [C II] using
Herschel HIFI on-the-fly strip maps. We use the [N II] spectra along with a
radiative transfer model to calculate the electron density of the gas and the
[C II] maps to illuminate the morphology of the ionized gas and model the
column density of CO-dark H2. We detect two [C II] and [N II] velocity
components, one along the line of sight to a CO molecular cloud at -207 km/s
associated with Sgr E and the other at -174 km/s outside the edge of another CO
cloud. From the [N II] emission we find that the average electron density is in
the range of about 5 to 25 cm{-3} for these features. This electron density is
much higher than that of the warm ionized medium in the disk. The column
density of the CO-dark H layer in the -207 km/s cloud is about 1-2X10{21}
cm{-2} in agreement with theoretical models. The CMZ extends further out in
Galactic radius by 7 to 14 pc in ionized gas than it does in molecular gas
traced by CO. The edge of the CMZ likely contains dense hot ionized gas
surrounding the neutral molecular material. The high fractional abundance of N+
and high electron density require an intense EUV field with a photon flux of
order 1e6 to 1e7 photons cm{-2} s{-1}, and/or efficient proton charge exchange
with nitrogen, at temperatures of order 1e4 K, and/or a large flux of X-rays.
Sgr E is a region of massive star formation which are a potential sources of
the EUV radiation that can ionize the gas. In addition X-ray sources and the
diffuse X-ray emission in the CMZ are candidates for ionizing nitrogen.Comment: 12 pages, 9 figure
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