Summary of research achievement in 2014–2018
Hsi-Wei Yen
My research is on star and planet formation. In order to understand the origin of our solar system
and the diverse properties of exo-planetary systems, it is essential to observationally study when and
where planets form in protoplanetary disks. Although protoplanetary disks are often observed around
young stellar objects, such as T Tauri or Herbig Ae/Be stars (Williams & Cieza 2011), it remains
unclear as to how and when such disks form during the process of star formation, and whether they
possess forming planets or not. Therefore, my research projects in the past five years are aiming
to understand the formation of protostellar disks, to reveal the evolution from protostellar disks to
protoplanetary disks, and to search for forming planets embedded in protoplanetary disks.
Formation and evolution of protostellar disks are closely related to the transfer of mass and
angular momentum from their natal dense cores on a 0.1 pc scale to the vicinity of protostars
on a 10 au scale (e.g., Terebey et al. 1984, Basu 1998). Thus, I have observed and characterized a
sample of protostellar disks embedded in dense cores and studied gas kinematics on scales from dense
cores to disks with single-dish and interferometric observations in molecular lines at (sub-)millimeter
wavelengths.
To find the signposts of forming planets embedded in protoplanetary disks, I have been developing analysis tools to enhance signal-to-noise (S/N) ratios of molecular-line data of protoplanetary
disks obtained with the Atacama Large Millimeter/submillimeter Array (ALMA) and to measure
disk structures at higher accuracy.
During the past five years, I published nine refereed journal papers as a first author on these
topics. Below I summarize the main results from my research projects in 2014–2018.
• Gas kinematics from 0.1 pc to 10 au scales
To study the transfer of angular momentum from dense cores to protostellar disks, I have observed
a sample of protostellar sources with ALMA to probe their inner protostellar envelopes on a scale
of tens of au (Yen et al. 2015b, 2017a). Together with data obtained with single-dish and other
interferometric observations in my earlier studies, I study the gas kinematics from large to small scales
in protostellar sources (Fig. 1) and mass accretion onto protoplanetary disks (Yen et al. 2014, 2017b).
With the observed velocity features in molecular-line emission, I constructed kinematical models
including LTE radiative transfer calculations to fit the observational data, and I measured infalling
and rotational velocities from large to small scales in protostellar sources. With the kinematical
information, I search for protostellar disks and measure the distribution of angular momentum in
protostellar sources. More detailed results are described below.
(b) SMT
• Gas kinematics from 0.1 pc to 30 AU
(c) SMA
(a) Optical
SMT
SMA
ALMA
(e) ALMA
(d) ALMA
~20000 AU
Gålfalk & Olofsson 2007
100 AU
Yen et al. 2010, 2011, 2015b
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Figure 1: (a) Optical image of
one of my targets, the Class 0
protostar B335. The image is
from Gâlfalk & Olofsson (2007).
Squares show the regions probed
with my SMT, SMA, and ALMA
observations. (b)–(e) Molecularline data that I obtained with the
SMT, SMA, and ALMA to study
the gas kinematics from 0.1 pc
to inner 30 AU scales. Contours
show the distributions of the integrated intensity, and color scales
show the mean velocity maps.
• Evolution of protostellar disks
I have developed an analytical method to identify protostellar disks embedded in dense cores by
measuring rotational velocities of gas motions as a function of radius around protostars. This method
has been adopted by my collaborators in several of their papers (Ohashi et al. 2014; Aso et al. 2015,
2017). With this method, I have detected Keplerian disks around a sample of young protostars with
ALMA observations (Yen et al. 2014, 2015b, 2017a).
Figure 2 presents the current statistics of protostellar disks, which I compiled from my observations and the literature. By adding my measurements of three sources with stellar masses <0.1
M (Yen et al. 2017a), my results could suggest rapid disk growth from the Class 0 to I stages,
as expected in the conventional model of collapsing dense cores (Terebey et al. 1984), and by the
end of the Class 0 stage the mass and size of protostellar disks are already comparable to those of
protoplanetary disks observed around T Tauri and Herbig Ae/Be stars. These results show that the
key stage of disk growth is likely when protostellar mass is between <0.1 M to 0.2 M . In addition,
my results showing the small disks at the early stage could suggest that the magnetic field suppresses
the disk formation (Yen et al. 2015b, 2017a).
Figure 2: Protostellar mass versus disk radius
for a sample of protostars. Blue diamonds and
red squares present the Class I and 0 protostars,
which are more and less evolved protostars, respectively. The results show that the Keplerian
disk radius quickly grows from <10 au to ∼100
au as the protostellar mass increases from <0.1
M to ∼0.2 M . Dark and light green lines
denote the scaling relations between protostellar
mass and disk radius in the collapse models in
Terebey et al. (1984) and Basu (1998), respectively.
Class 0
Class I
Rd~M*
Rd~M*3
unresolved, inferred disks
• Angular momentum transfer from dense cores to disks
I have measured the radial profiles of the specific angular momentum in a sample of protostars with
SMA, ALMA, and single-dish observations (Yen et al. 2017a). These measurements can be compared
with theoretical models to study the angular momentum transfer from dense cores to disks around
protostars. I computed theoretical profiles at different evolutionary stages and compared with my
measurements and those from the literature (Fig. 3). My results suggest that the overall evolution of
the angular momentum transfer can be described with the conventional inside-out collapse models,
where the angular momentum is conserved. Nevertheless, possible signs of the magnetic field effects,
which transfer the angular momentum outward, are seen in individual sources (Yen et al. 2017a,
2018a).
• Enhancing S/N ratios — velocity aligned stacking method
To enhance S/N ratios of molecular-line data of protoplanetary disks, I have developed a new method
by taking advantage of the Keplerian rotational velocity pattern (Yen et al. 2016b). My method aligns
spectra according to their different rotational velocities at their different positions in a disk and stacks
them. This method can be applied to search for faint molecular lines, to measure intensity profiles
of molecular lines more accurately, and to constrain stellar mass and disk orientation, even if a disk
cannot be directly detected in molecular lines (Fig. 4a).
I have applied this method to measure dynamical masses of a sample of T Tauri stars (Fig. 4b).
By using this method, each measurement of dynamical stellar mass in Fig. 4b only took an on-source
integration time of one minute with ALMA even for faint protoplanetary disks with integrated 13 CO
fluxes as low as 200–300 mJy km s−1 (Yen et al. 2018b). This can be achieved because of the
enhancement of the S/N ratios by this method. In contrast, when measuring dynamical stellar mass
with the conventional method of fitting disk models to observed velocity channel maps, an integration
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Figure 3: Radial profiles of specific angular momenta. Filled and open circles display measured
envelope and disk rotations in a sample of protostars, respectively. Each color represents a different
protostar (inlet). Grey dashed curves show the expected profiles computed from an inside-out collapse
of rigid rotating dense cores where the angular momentum is conserved, for protostellar masses of 1.0,
0.4, and 0.1 M from top to bottom. Numbers in
colors label the measured protostellar masses of the
sample protostars, as an evolutionary indicator for
comparison with the expected profiles.
1.0
1.8
0.4
0.6
0.5
0.3
0.2
0.1
0.01
0.03
0.05
HH 111
TMC-1A
L1551 IRS 5
HH 212
L1527
IRAS 16253
IRAS 15398
B335
time that is a factor of five to ten longer is needed to achieve sufficient S/N ratios for the model
fitting to those faint disks. Therefore, this method can be applied to measure dynamical masses of a
large sample of young stars with a reasonable amount of observing time to study stellar evolution.
(a)
(b)
H2CO (32,1-22,0)
original data
my method
Figure 4: (a) H2 CO spectrum of the protoplanetary disk around HD 163296. Black and red histograms
present the spectra from the original data and after applying my method, respectively (Yen et al. 2016b). (b)
Stellar masses of T Tauri stars in the Lupus clouds estimated from the spectroscopic information with the
stellar evolutionary models (horizontal axis; Alcalá et al. 2014, 2017) and from the dynamical information
traced by the 13 CO emission with my method (vertical axis). Black, green, and red dashed lines denote the
regions where the differences in mass are 0, 25%, and 50%, respectively. The dynamical mass measured with
my method is consistent with the spectroscopic mass within the uncertainty.
• Fine disk structures — signs of gas gaps in
the HL Tau disk
The ALMA observations discovered a series of gaps in
the submm and mm continuum emission in the disk
around HL Tau (ALMA partnership et al. 2015), but
the gas counterparts of those dust gaps were not detected due to the limited S/N ratio. I have further
analyzed the same archival data and found the signs
of the gas gaps coincident with the dust gaps (Fig. 5;
Yen et al. 2016a). My results suggest that material,
including both dust and gas, is likely depleted at these
radii. These results provide additional constraints on
the existence of forming planets, and the depths and
widths of the gas gaps derived from my analysis are
consistent with the expectation from the gaps opened
Figure 5: ALMA images of the HCO+ gas (blue)
by forming Jovian mass planets.
and dust (red) distributions in the HL Tau disk.
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Reference
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