90 research outputs found
Magnetic fields and young stellar objects in cometary cloud LDN 1616
LDN 1615/1616 and CB 28 (hereafter, L1616) together form a cometary globule
located at an angular distance of about 8 degrees west of the Orion OB1
association, aligned roughly along the east-west direction, and showing a
distinct head-tail structure. The presence of massive stars in the Orion belt
has been considered to be responsible for the radiation driven implosion mode
of star formation in L1616. Based on the latest Gaia EDR3 measurements of the
previously known young stellar objects (YSOs) associated with L1616, we find
the distance to this cloud of 3845 pc. We present optical polarimetry
towards L1616 that maps the plane-of-sky component of the ambient magnetic
field (B) geometry. Based on the proper motion of the YSOs associated
with L1616, we investigate their plane-of-sky motion relative to the exciting
star Ori. Using the Gaia EDR3 measurements of the distances and
proper motions of the YSOs, we find two additional sources comoving with the
known YSOs. One comoving source is HD33056, a B9 star and the other might be a
young pre-main sequence star not reported in previous studies. The mean
direction of B is found to follow the cloud structure. This could be
the effect of dragging of the magnetic field lines by the impact of the
ionizing radiation from Ori. Based on the pressure exerted on L1616,
and the ages of the associated YSOs, we show that it could possibly be the main
source of ionization in L1616, and thus the star formation in it
TRAO Survey of Nearby Filamentary Molecular clouds, the Universal Nursery of Stars (TRAO FUNS) I. Dynamics and Chemistry of L1478 in the California Molecular Cloud
"TRAO FUNS" is a project to survey Gould Belt's clouds in molecular lines.
This paper presents its first results on the central region of the California
molecular cloud, L1478. We performed On-The-Fly mapping observations using the
Taedeok Radio Astronomy Observatory (TRAO) 14m single dish telescope equipped
with a 16 multi-beam array covering 1.0 square degree area of this region
using CO (1-0) mainly tracing low density cloud and about 460 square
arcminute area using NH (1-0) mainly tracing dense cores. CS (2-1)
and SO were also used simultaneously to map 440 square
arcminute area of this region. We identified 10 filaments by applying the
dendrogram technique to the CO data-cube and 8 dense NH
cores by using {\sc FellWalker}. Basic physical properties of filaments such as
mass, length, width, velocity field, and velocity dispersion are derived. It is
found that L1478 consists of several filaments with slightly different
velocities. Especially the filaments which are supercritical are found to
contain dense cores detected in NH. Comparison of non-thermal
velocity dispersions derived from CO and NH for the
filaments and dense cores indicates that some of dense cores share similar
kinematics with those of the surrounding filaments while several dense cores
have different kinematics with those of their filaments. This suggests that the
formation mechanism of dense cores and filaments can be different in individual
filaments depending on their morphologies and environments.Comment: 25 pages, 15 figures, accepted for publication in Ap
On the collisional disalignment of dust grains in illuminated and shaded regions of IC 63
Interstellar dust grain alignment causes polarization from UV to mm wavelengths, allowing the study of the geometry and strength of the magnetic field. Over the last couple of decades, observations and theory have led to the establishment of the radiative alignment torque mechanism as a leading candidate to explain the effect. With a quantitatively well constrained theory, polarization can be used not only to study the interstellar magnetic field, but also the dust and other environmental parameters. Photodissociation regions, with their intense, anisotropic radiation fields, consequent rapid H2 formation, and high spatial density-contrast provide a rich environment for such studies. Here we discuss an expanded optical, NIR, and mm-wave study of the IC 63 nebula, showing strong H2 formation-enhanced alignment and the first direct empirical evidence for disalignment due to gas-grain collisions using high-resolution HCO+(J = 1-0) observations. We find that a relative amount of polarization is marginally anticorrelated with column density of HCO+. However, separating the lines of sight of optical polarimetry into those behind, or in front of, a dense clump as seen from γ Cas, the distribution separates into two well defined sets, with data corresponding to "shaded" gas having a shallower slope. This is expected if the decrease in polarization is caused by collisions since collisional disalignment rate is proportional to RC∝nT" role="presentation">RC∝nT−−√ . Ratios of the best-fit slopes for the "illuminated" and "shaded" samples of lines of sight agrees, within the uncertainties, with the square root of the two-temperature H2 excitation in the nebula seen by Thi et al.Fil: Soam, Archana. Sofia Science Center; Estados UnidosFil: Anderson, B. G. Sofia Science Center; Estados UnidosFil: Acosta Pulido, Jose. Instituto de Astrofisica de Canarias; EspañaFil: Fernandez Lopez, Manuel. Provincia de Buenos Aires. Gobernación. Comisión de Investigaciones Científicas. Instituto Argentino de Radioastronomía. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata. Instituto Argentino de Radioastronomía; ArgentinaFil: Vaillancourt, J. E.. Lincoln Laboratory; Estados UnidosFil: Widicus Weaver, S. L.. Department Of Chemistry; Estados UnidosFil: Piirola, V.. University Of Turku; FinlandiaFil: Gordon, M. S.. Sofia Science Center; Estados Unido
First Sub-parsec-scale Mapping of Magnetic Fields in the Vicinity of a Very-low-luminosity Object, L1521F-IRS
L1521F is found to be forming multiple cores and it is cited as an example of the densest core with an embedded VeLLO in a highly dynamical environment. We present the core-scale magnetic fields (B-fields) in the near vicinity of the VeLLO L1521F-IRS using submillimeter polarization measurements at 850 mu m using JCMT POL-2. This is the first attempt to use high-sensitivity observations to map the sub-parsec-scale B-fields in a core with a VeLLO. The B-fields are ordered and very well connected to the parsec-scale field geometry seen in our earlier optical polarization observations and the large-scale structure seen in Planck dust polarization. The core-scale B-field strength estimated using the Davis-Chandrasekhar-Fermi relation is 330 +/- 100 mu G, which is more than 10 times the value we obtained in the envelope (the envelope in this paper is the "core envelope"). This indicates that B-fields are getting stronger on smaller scales. The magnetic energies are found to be 1 to 2 orders of magnitude higher than nonthermal kinetic energies in the envelope and core. This suggests that magnetic fields are more important than turbulence in the energy budget of L1521F. The mass-to-flux ratio of 2.3 +/- 0.7 suggests that the core is magnetically supercritical. The degree of polarization is steadily decreasing toward the denser part of the core with a power-law slope of -0.86.Peer reviewe
The origin of dust polarization in the Orion Bar
The linear polarization of thermal dust emission provides a powerful tool to
probe interstellar and circumstellar magnetic fields, because aspherical grains
tend to align themselves with magnetic field lines. While the Radiative
Alignment Torque (RAT) mechanism provides a theoretical framework to this
phenomenon, some aspects of this alignment mechanism still need to be
quantitatively tested. One such aspect is the possibility that the reference
alignment direction changes from the magnetic field ("B-RAT") to the radiation
field k-vector ("k-RAT") in areas of strong radiation fields. We investigate
this transition toward the Orion Bar PDR, using multi-wavelength SOFIA HAWC+
dust polarization observations. The polarization angle maps show that the
radiation field direction is on average not the preferred grain alignment axis.
We constrain the grain sizes for which the transition from B-RAT to k-RAT occur
in the Orion Bar (grains > 0.1 {\mu}m toward the most irradiated locations),
and explore the radiatively driven rotational disruption that may take place in
the high-radiation environment of the Bar for large grains. While the grains
susceptible to rotational disruption should be in supra-thermal rotation and
aligned with the magnetic field, k-RAT aligned grains would rotate at thermal
velocities. We find that the grain size at which the alignment shifts from
B-RAT to k-RAT corresponds to grains too large to survive the rotational
disruption. Therefore, we expect a large fraction of grains to be aligned at
supra-thermal rotation with the magnetic field, and potentially be subject to
rotational disruption depending on their tensile strength
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