94 research outputs found
Search for Remnant Clouds Associated with the TW Hya Association
We report on a search for the parental molecular clouds of the TW Hya
association (TWA), using CO emission and Na I absorption lines. TWA is the
nearest young (~ 50 pc; ~ 10 Myr) stellar association, yet in spite of its
youth, there are no detection of any associated natal molecular gas, as is the
case for other typical young clusters. Using infrared maps as a guide, we
conducted a CO cloud survey toward a region with a dust extinction of E(B-V) >
0.2 mag, or AV > 0.6 mag. CO emission is detected toward three IR dust clouds,
and we reject one cloud from the TWA, as no interstellar Na absorption was
detected from the nearby Hipparcos stars, implying that it is too distant to be
related. The other two clouds exhibit only faint and small-scale CO emission.
Interstellar Na I absorptions of Hipparcos targets, HIP 57809, HIP 64837, and
HIP 64925 (at distances of 133, 81, and 101 pc, respectively) by these couds is
also detected. We conclude that only a small fraction of the interstellar
matter (ISM) toward the IR dust cloud is located at distance less than 100 pc,
which may be all that is left of the remnant clouds of TWA; the remaining
remnant cloud having dissipated in the last ~ 1 Myr. Such a short dissipation
timescale may be due to an external perturbation or kinematic segregation that
has a large stellar proper motion relative to the natal cloud.Comment: 7 pages, 5 figures. PASJ accepte
Toward Understanding the Origin of Turbulence in Molecular Clouds: Small Scale Structures as Units of Dynamical Multi-Phase Interstellar Medium
In order to investigate the origin of the interstellar turbulence, detailed
observations in the CO J=1--0 and 3--2 lines have been carried out in an
interacting region of a molecular cloud with an HII region. As a result,
several 1,000 to 10,000 AU scale cloudlets with small velocity dispersion are
detected, whose systemic velocities have a relatively large scatter of a few
km/s. It is suggested that the cloud is composed of small-scale dense and cold
structures and their overlapping effect makes it appear to be a turbulent
entity as a whole. This picture strongly supports the two-phase model of
turbulent medium driven by thermal instability proposed previously. On the
surface of the present cloud, the turbulence is likely to be driven by thermal
instability following ionization shock compression and UV irradiation. Those
small scale structures with line width of ~ 0.6 km/s have a relatively high CO
line ratio of J=3--2 to 1--0, 1 < R(3-2/1-0) < 2. The large velocity gradient
analysis implies that the 0.6 km/s width component cloudlets have an average
density of 10^{3-4} cm^{-3}, which is relatively high at cloud edges, but their
masses are only < 0.05 M_{sun}.Comment: 12 pages, 9 figures. To be published in the Astrophysical Journa
Properties of star formation of the Large Magellanic Cloud as probed by young stellar objects
We perform a systematic study of evolutionary stages and stellar masses of
young stellar objects (YSOs) in the Large Magellanic Cloud (LMC) to investigate
properties of star formation of the galaxy. There are 4825 sources in our YSO
sample, which are constructed by combining the previous studies identifying
YSOs in the LMC. Spectral energy distributions of the YSOs from optical to
infrared wavelengths were fitted with a model consisting of stellar, polycyclic
aromatic hydrocarbon and dust emissions. We utilize the stellar-to-dust
luminosity ratios thus derived to study the evolutionary stages of the sources;
younger YSOs are expected to show lower stellar-to-dust luminosity ratios. We
find that most of the YSOs are associated with the interstellar gas across the
galaxy, which are younger with more gases, suggesting that more recent star
formation is associated with larger amounts of the interstellar medium (ISM).
N157 shows a hint of higher stellar-to-dust luminosity ratios between active
star-forming regions in the LMC, suggesting that recent star formation in N157
is possibly in later evolutionary stages. We also find that the stellar mass
function tends to be bottom-heavy in supergiant shells (SGSs), indicating that
gas compression by SGSs may be ineffective in compressing the ISM enough to
trigger massive star formation. There is no significant difference in the
stellar mass function between YSOs likely associated with the interface between
colliding SGSs and those with a single SGS, suggesting that gas compression by
collisions between SGSs may also be ineffective for massive star formation.Comment: 26 pages, 16 figures, accepted for publication in Ap
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