Papers - MASUNAGA, Hirohiko

Infall Signatures in Molecular Line Spectra of Protostellar Envelopes. Reviewed

Masunaga, H. and Inutsuka, S.

Astrophys. J.   Vol. 536   page： 406-415   2000

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Authorship：Lead author   Language：English   Publishing type：Research paper (scientific journal)  

A double-peaked proÐle with a stronger blue peak in a molecular line spectrum is considered strong
evidence for the infall motion in the gas envelope surrounding a protostar. Some past studies performed
model calculations for reproducing observed spectral proÐles using simpliÐed dynamical models such as
isothermal similarity solutions. However, validity of the similarity solutions for spectral line synthesis
should be examined in comparison with more realistic dynamical models.
In this paper we carry out theoretical modeling of molecular line spectra, adopting a spherically symmetric
radiation hydrodynamical model for protostar formation taken from a recent work by the
authors. This study concentrates on how infall motions could account for the asymmetric line proÐles
observed toward protostellar sources. We do not explicitly consider the e†ects of rotation and outÑows
on the generation of line proÐles. In our numerical code for the non-LTE line transfer, the level populations
are fully consistent with the radiation Ðeld under an arbitrary physical structure in spherical symmetry,
and hence, reliable spectral synthesis has been enabled and compared to the LTE, large velocity
gradient (or Sobolev), and microturbulence approximations.
Contrary to the remarks by Zhou, we do not Ðnd an overestimation of line widths, although the
dynamical evolution resembles the Larson-Penston solution rather than the expansion-wave solution.
Furthermore, the infall motion produces wings extending to v\^2 km s~1 in line spectra, in contrast
to previous works where wings could not be produced by infall models. These results imply that simpli-
Ðed infall models, such as the isothermal similarity solutions adopted by previous authors, are not
always suitable to the detailed modeling of line spectra.

Does "t @ 1" Terminate the Isothermal Evolution of Collapsing Clouds? Reviewed

Masunaga, H. and Inutsuka, S.

Astrophys. J.   Vol. 510   page： 822-827   1999

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Authorship：Lead author   Language：English   Publishing type：Research paper (scientific journal)  

We examine when gravitationally collapsing clouds terminate their isothermal evolution. According to
our previous work, the condition with which isothermality is broken down is classiÐed into three cases,
i.e., when (1) the compressional heating rate overtakes the thermal cooling rate, (2) the optical depth for
thermal radiation reaches unity, or (3) the compressional heating rate becomes comparable with the
energy transport rate because of radiative di†usion. In the present paper this classiÐcation is extended to
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that the condition of ““qB1 ÏÏ never terminates isothermality, but nonisothermal evolutions begin either
earlier or later depending on the initial temperature and opacity. This result contrasts with the conventional
idea that opaqueness breaks isothermality. On the basis of the critical density discussed above, the
minimum Jeans mass for fragmentation, MF, is reconsidered. In contrast to the results by previous
authors that MF is insensitive to Tinit and i, we Ðnd that MF can be substantially larger than the typical
value of D10~2 M depending on and i. In particular, increases with decreasing metallicity, _ Tinit MF
MFPi~1, for low-metal clouds. A cloud with i\10~4 cm2 g~1 and Tinit\10 K yields MF\3.7 M_.
Finally, our critical densities would be helpful for hydrodynamic simulations that are intended to simply
handle the hardening of the equation of state.

A Radiation Hydrodynamic Model for Protostellar Collapse I. The First Collapse. Reviewed

Masunaga, H., Miyama, S.M., and Inutsuka, S.

Astrophys. J.   Vol. 495   page： 346-369   1998

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Authorship：Lead author   Language：English   Publishing type：Research paper (scientific journal)  

Dynamical collapse of a molecular cloud core and the formation of a star are investigated by performing
radiation hydrodynamic calculations in spherical symmetry. The angle-dependent and
frequency-dependent radiative transfer equation is solved without any di†usion approximations, and the
evolution of the spectral energy distribution (SED) is examined.
In the present paper, as the Ðrst step in a series of our work, evolutions before hydrogen molecules
begin to dissociate (the so-called Ðrst collapse) are examined for di†erent masses and initial temperatures
of the parent cloud cores and for di†erent opacities. Numerical results for a typical case [Tinit\10 K
and iP(10 K)D0.01 cm2 g~1] show that the radius and mass of the Ðrst core are D5 AU and D0.05
M respectively. These values are independent both of the mass of the parent cloud core and of the _,
initial density proÐle. The analytical expressions for the radius, mass, and accretion luminosity of the
Ðrst core are also obtained. The SED contains only cold components of a few times 10 K throughout
the Ðrst collapse phase, because the opaque envelope veils the Ðrst core from observers. We suggest that
the molecular cloud cores with luminosities higher than D0.1 L should contain young protostars deep _
in the center, even if they show no evidence for the existence of central stars in near-infrared and optical
observations.