71 research outputs found
High-dynamic-range extinction mapping of infrared dark clouds: Dependence of density variance with sonic Mach number in molecular clouds
Measuring the mass distribution of infrared dark clouds (IRDCs) over the wide
dynamic range of their column densities is a fundamental obstacle in
determining the initial conditions of high-mass star formation and star cluster
formation. We present a new technique to derive high-dynamic-range,
arcsecond-scale resolution column density data for IRDCs and demonstrate the
potential of such data in measuring the density variance - sonic Mach number
relation in molecular clouds. We combine near-infrared data from the
UKIDSS/Galactic Plane Survey with mid-infrared data from the Spitzer/GLIMPSE
survey to derive dust extinction maps for a sample of ten IRDCs. We then
examine the linewidths of the IRDCs using 13CO line emission data from the
FCRAO/Galactic Ring Survey and derive a column density - sonic Mach number
relation for them. For comparison, we also examine the relation in a sample of
nearby molecular clouds. The presented column density mapping technique
provides a very capable, temperature independent tool for mapping IRDCs over
the column density range equivalent to A_V=1-100 mag at a resolution of 2".
Using the data provided by the technique, we present the first direct
measurement of the relationship between the column density dispersion,
\sigma_{N/}, and sonic Mach number, M_s, in molecular clouds. We detect
correlation between the variables with about 3-sigma confidence. We derive the
relation \sigma_{N/} = (0.047 \pm 0.016) Ms, which is suggestive of the
correlation coefficient between the volume density and sonic Mach number,
\sigma_{\rho/} = (0.20^{+0.37}_{-0.22}) Ms, in which the quoted
uncertainties indicate the 3-sigma range. When coupled with the results of
recent numerical works, the existence of the correlation supports the picture
of weak correlation between the magnetic field strength and density in
molecular clouds (i.e., B ~ \rho^{0.5}).Comment: Accepted for publication in A&A. 29 pages. Download the version with
full-resolution figures from
http://www.mpia-hd.mpg.de/homes/jtkainul/NexusI/PaperII_arxiv.pdf.g
Studies of the star-forming structures in the dense interstellar medium : a view by dust extinction
New stars in galaxies form in dense, molecular clouds of the interstellar medium. Measuring how the mass is distributed in these clouds is of crucial importance for the current theories of star formation. This is because several open issues in them, such as the strength of different mechanism regulating star formation and the origin of stellar masses, can be addressed using detailed information on the cloud structure. Unfortunately, quantifying the mass distribution in molecular clouds accurately over a wide spatial and dynamical range is a fundamental problem in the modern astrophysics.
This thesis presents studies examining the structure of dense molecular clouds and the distribution of mass in them, with the emphasis on nearby clouds that are sites of low-mass star formation. In particular, this thesis concentrates on investigating the mass distributions using the near infrared dust extinction mapping technique. In this technique, the gas column densities towards molecular clouds are determined by examining radiation from the stars that shine through the clouds. In addition, the thesis examines the feasibility of using a similar technique to derive the masses of molecular clouds in nearby external galaxies.
The papers presented in this thesis demonstrate how the near infrared dust extinction mapping technique can be used to extract detailed information on the mass distribution in nearby molecular clouds. Furthermore, such information is used to examine characteristics crucial for the star formation in the clouds. Regarding the use of extinction mapping technique in nearby galaxies, the papers of this thesis show that deriving the masses of molecular clouds using the technique suffers from strong biases. However, it is shown that some structural properties can still be examined with the technique.Galaksien uudet tÀhdet syntyvÀt tÀhtienvÀlisen avaruuden tiheissÀ kaasupilvissÀ joita kutsutaan molekyylipilviksi. NÀiden molekyylipilvien rakenteen mÀÀrittÀminen on tÀhtien syntyteorioiden kannalta erittÀin tÀrkeÀÀ, koska monet teorioihin liittyvÀt fysikaaliset ilmiöt heijastuvat suoraan pilvien rakenteeseen. Molekyylipilvien rakenteen tarkka mÀÀritys on kuitenkin ongelmallista, sillÀ kaasua josta pilvet koostuvat on verrattaen vaikea havaita.
Tutkin vÀitöskirjatyössÀni tÀhtien syntyprosessin alkuhetkiÀ tarkastelemalla kaasun muodostamia rakenteita molekyylipilvissÀ. TyössÀ keskitytÀÀn soveltamaan uutta ns. lÀhi-infrapuna-alueen vÀrieksessi-menetelmÀÀ, jossa pilven rakenne mÀÀritetÀÀn tarkastelemalla sen lÀpi loistavien tÀhtien sÀteilyÀ. MenetelmÀn soveltamisen lisÀksi työssÀ tutkitaan menetelmÀn tarkkuutta ja mahdollisia sovellusalueita.
VÀitöskirjatyöni tutkimukset havainnollistavat kuinka vÀriksessi-menetelmÀllÀ voidaan saavuttaa verrattaen tarkka ja nÀin ollen erittÀin hyödyllinen nÀkymÀ tÀhtien syntyrakenteisiin molekyylipilvissÀ. LisÀksi tutkimuksissa tarkastellaan erilaisten rakenteiden merkitystÀ nykyisille tÀhtien syntyteorioille. Tutkimuksissa selvitetÀÀn myös millÀ tarkkuudella menetelmÀÀ voidaan soveltaa Linnunradan ulkopuolisten galaksien molekyylipilvien tutkimukseen
Connection between dense gas mass fraction, turbulence driving, and star formation efficiency of molecular clouds
We examine the physical parameters that affect the accumulation of gas in
molecular clouds to high column densities where the formation of stars takes
place. In particular, we analyze the dense gas mass fraction (DGMF) in a set of
self-gravitating, isothermal, magnetohydrodynamic turbulence simulations
including sink particles to model star formation. We find that the simulations
predict close to exponential DGMFs over the column density range N(H2) = 3-25 x
10^{21} cm^{-2} that can be easily probed via, e.g., dust extinction
measurements. The exponential slopes correlate with the type of turbulence
driving and also with the star formation efficiency. They are almost
uncorrelated with the sonic Mach number and magnetic-field strength. The slopes
at early stages of cloud evolution are steeper than at the later stages. A
comparison of these predictions with observations shows that only simulations
with relatively non-compressive driving (b ~< 0.4) agree with the DGMFs of
nearby molecular clouds. Massive infrared dark clouds can show DGMFs that are
in agreement with more compressive driving. The DGMFs of molecular clouds can
be significantly affected by how compressive the turbulence is on average.
Variations in the level of compression can cause scatter to the DGMF slopes,
and some variation is indeed necessary to explain the spread of the observed
DGMF slopes. The observed DGMF slopes can also be affected by the clouds' star
formation activities and statistical cloud-to-cloud variations.Comment: 7 pages, 7 figures, accepted to A&A Letter
Three-dimensional Shape Explains Star Formation Mystery of California and Orion A
The new Gaia data release (EDR3) with improved astrometry has opened a new era in studying our Milky Way in fine detail. We use Gaia EDR3 astrometry together with 2MASS and WISE photometry to study two of the most massive molecular clouds in the solar vicinity: Orion A and California. Despite having remarkable similarities in the plane of the sky in terms of shape, size, and extinction, California has an order of magnitude lower star formation efficiency. We use our state-of-the-art dust mapping technique to derive the detailed three-dimensional (3D) structure of the two clouds, taking into account both distance and extinction uncertainties, and a full 3D spatial correlation between neighboring points. We discover that, despite the apparent filamentary structure in the plane of the sky, California is a flat 120 pc-long sheet extending from 410 to 530 pc. We show that not only Orion A and California differ substantially in their 3D shapes, but also Orion A has considerably higher density substructures in 3D than California. This result presents a compelling reason why the two clouds have different star formation activities. We also demonstrate how the viewing angle of California can substantially change the cloud\u27s position in the Kennicutt-Schmidt relation. This underlines the importance of 3D information in interpreting star formation relations and challenges studies that rely solely on the column density thresholds to determine star formation activities in molecular clouds. Finally, we provide accurate distance estimates to multiple lines of sight toward various parts of the two clouds
Structure and Fragmentation of a high line-mass filament: Nessie
An increasing number of hundred-parsec scale, high line-mass filaments have
been detected in the Galaxy. Their evolutionary path, including fragmentation
towards star formation, is virtually unknown. We characterize the fragmentation
within the Nessie filament, covering size-scales between 0.1-100 pc. We
also connect the small-scale fragments to the star-forming potential of the
cloud. We combine near-infrared data from the VVV survey with mid-infrared
GLIMPSE data to derive a high-resolution dust extinction map and apply a
wavelet decomposition technique on it to analyze the fragmentation
characteristics of the cloud, which are compared with predictions from
fragmentation models. We compare the detected objects to those identified in
10 times coarser resolution from ATLASGAL data. We present a
high-resolution extinction map of Nessie. We estimate the mean line-mass of
Nessie to be 627 M/pc and the distance to be 3.5 kpc. We
find that Nessie shows fragmentation at multiple size scales. The
nearest-neighbour separations of the fragments at all scales are within a
factor of 2 of the Jeans' length at that scale. However, the relationship
between the mean densities of the fragments and their separations is
significantly shallower than expected for Jeans' fragmentation. The
relationship is similar to the one predicted for a filament that exhibits a
Larson-like scaling between size-scale and velocity dispersion; such a scaling
may result from turbulent support. Based on the number of YSOs in Nessie, we
estimate that the star formation rate is 371 M/Myr; similar
values result if using the number of dense cores, or the amount of dense gas,
as the proxy of star formation. The star formation efficiency is 0.017. These
numbers indicate that Nessie's star-forming content is comparable to the Solar
neighborhood giant molecular clouds like Orion A
The Darkest Shadows: Deep Mid-Infrared Extinction Mapping of a Massive Protocluster
We use deep Spitzer-IRAC imaging of a massive Infrared Dark Cloud
(IRDC) G028.37+00.07 to construct a Mid-Infrared (MIR) extinction map that
probes mass surface densities up to
(mag), amongst the highest values yet probed by extinction
mapping. Merging with a NIR extinction map of the region, creates a high
dynamic range map that reveals structures down to mag. We utilize
the map to: (1) Measure a cloud mass within a radius
of pc. CO kinematics indicate that the cloud is gravitationally
bound. It thus has the potential to form one of the most massive young star
clusters known in the Galaxy. (2) Characterize the structures of 16 massive
cores within the IRDC, finding they can be fit by singular polytropic spheres
with and . They have
--- relatively low values
that, along with their measured cold temperatures, suggest magnetic fields,
rather than accretion-powered radiative heating, are important for controlling
fragmentation of these cores. (3) Determine the (equivalently column
density or ) probability distribution function (PDF) for a region that is
near complete for mag. The PDF is well fit by a single log-normal with
mean mag, high compared to other known clouds. It does
not exhibit a separate high-end power law tail, which has been claimed to
indicate the importance of self-gravity. However, we suggest that the PDF does
result from a self-similar, self-gravitating hierarchy of structure being
present over a wide range of scales in the cloud.Comment: 6 pages, 3 figures, 1 table, accepted to ApJ
Deep point spread function photometric catalog of the VVV survey data
Context. The Vista Variables in the Via Lactea (VVV) survey has performed a multi-epoch near-infrared imaging of the inner Galactic plane. High-fidelity photometric catalogs are needed to utilize the data. Aims. We aim at producing a deep, point spread function (PSF) photometric catalog for the VVV survey J-,H-, and K-s-band data. Specifically, we aim to take advantage of multiple epochs of the survey to reach high limiting magnitudes. Methods. We developed an automatic PSF-fitting pipeline based on the DaoPHOT algorithm and performed photometry on the stacked VVV images in J,H, and K-s bands. Results. We present a PSF photometric catalog in the Vega system that contains about 926 million sources in the J,H, and K-s filters. About 10% of the sources are flagged as possible spurious detections. The 5 sigma limiting magnitudes of the sources with high reliability are about 20.8, 19.5, and 18.7 mag in the J,H, and K-s bands, respectively, depending on the local crowding condition. Our photometric catalog reaches on average about one magnitude deeper than the previously released PSF DoPHOT photometric catalog and includes less spurious detections. There are significant differences in the brightnesses of faint sources between our catalog and the previously released one. The likely origin of these differences is in the different photometric algorithms that are used; it is not straightforward to assess which catalog is more accurate in different situations. Our new catalog is beneficial especially for science goals that require high limiting magnitudes; our catalog reaches such high magnitudes in fields that have a relatively uniform source number density. Overall, the limiting magnitudes and completeness are different in fields with different crowding conditions
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