We present the results of a systematic search for molecular outflows in 68 Very Low Luminosity Objects (VeLLOs) from single-dish observations in CO isotopologues, finding 16 VeLLOs that show clear ...outflow signatures in the CO maps. Together with an additional three VeLLOs from the literature, we analyzed the outflow properties for these 19 VeLLOs, identifying 15 VeLLOs as proto-brown-dwarf (proto-BD) candidates and 4 VeLLOs as likely faint protostar candidates. The proto-BD candidates are found to have a mass accretion rate (∼10−8-10−7 M yr−1) lower than that of the protostar candidates ( 10−6 M yr−1). Their accretion luminosities are similar to or smaller than their internal luminosities, implying that many proto-BD candidates might have had either small accretion activity in a quiescent manner throughout their lifetimes, or be currently exhibiting relatively higher (or episodic) mass accretion than in the past. Outflows of many proto-BDs show strong trends of being less active if they are fainter or have less massive envelopes. The outflow forces and internal luminosities for more than half of the proto-BD candidates seem to follow the evolutionary track of a protostar with an initial envelope mass of ∼0.08 M , indicating that some BDs may form in less massive dense cores in a similar way to normal stars. But, because there also exists a significant fraction (about 40%) of proto-BDs with a much weaker outflow force than expected from the relations for protostars, we should not rule out the possibility of other formation mechanism for BDs.
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 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 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.
We mapped two molecular cloud cores in the Orion A cloud with the 7 m Array of the Atacama Compact Array (ACA) of the Atacama Large Millimeter/submillimeterArray (ALMA) and with the Nobeyama 45 m ...radio telescope. These cores have bright N2D+ emission in single-pointing observations with the Nobeyama 45 m radio telescope, have a relatively high deuterium fraction, and are thought to be close to the onset of star formation. One is a star-forming core, and the other is starless. These cores are located along filaments observed in N2H+ and show narrow line widths of 0.41 km s−1 and 0.45 km s−1 in N2D+, respectively, with the Nobeyama 45 m telescope. Both cores were detected with the ALMA ACA 7 m Array in the continuum and molecular lines at Band 6. The starless core G211 shows a clumpy structure with several sub-cores, which in turn show chemical differences. Also, the sub-cores in G211 have internal motions that are almost purely thermal. The starless sub-core G211D, in particular, shows a hint of the inverse P Cygni profile, suggesting infall motion. The star-forming core G210 shows an interesting spatial feature of two N2D+ peaks of similar intensity and radial velocity located symmetrically with respect to the single dust continuum peak. One interpretation is that the two N2D+ peaks represent an edge-on pseudo-disk. The CO outflow lobes, however, are not directed perpendicular to the line connecting both N2D+ peaks.
We present the star formation rates (SFRs) of galaxies with redshifts of 0 <
z
< 6 and cosmic star formation rate densities as a function of redshift. The SFRs for the galaxies at infrared emission ...have been investigated for 1752 sources from the Spitzer 70 μm observations of the Spitzer COSMOS Legacy survey and the various surveys in the literature. Analyzing the SFRs of these objects provides the distribution of SFRs across the redshift range. We have derived the star formation rate densities (SFRDs) based on the present analysis of SFRs and comoving volume calculated from the cosmic parameters we have used. Our results show a slightly steeper slope at lower and intermediate redshift, 0 <
z
< 2. We have found a steep decline of SFRD from
z
=
1
to 0, but measured a peak of SFRD at
z
≈
2.2
−
2.4
.
Abstract
In this study, 36 cores (30 starless and six protostellar) identified in Orion were surveyed to search for inward motions. We used the Nobeyama 45 m radio telescope, and mapped the cores in ...the
J
= 1 → 0 transitions of HCO
+
, H
13
CO
+
, N
2
H
+
, HNC, and HN
13
C. The asymmetry parameter
δV
, which was the ratio of the difference between the HCO
+
and H
13
CO
+
peak velocities to the H
13
CO
+
line width, was biased toward negative values, suggesting that inward motions were more dominant than outward motions. Three starless cores (10% of all starless cores surveyed) were identified as cores with blue-skewed line profiles (asymmetric profiles with more intense blueshifted emission), and another two starless cores (7%) were identified as candidate blue-skewed line profiles. The peak velocity difference between HCO
+
and H
13
CO
+
of them was up to 0.9 km s
−1
, suggesting that some inward motions exceeded the speed of sound for the quiescent gas (∼10–17 K). The mean of
δV
of the five aforementioned starless cores was derived to be −0.5 ± 0.3. One core, G211.16−19.33North3, observed using the Atacama Compact Array of the Atacama Large Millimeter/submillimeter Array in DCO
+
J
= 3 → 2 exhibited blue-skewed features. Velocity offset in the blue-skewed line profile with a dip in the DCO
+
J
= 3 → 2 line was larger (∼0.5 km s
−1
) than that in HCO
+
J
= 1 → 0 (∼0.2 km s
−1
), which may represent gravitational acceleration of inward motions. It seems that this core is at the last stage in the starless phase, judging from the chemical evolution factor version 2.0 (CEF2.0).
Abstract
Prestellar cores are self-gravitating dense and cold structures within molecular clouds where future stars are born. They are expected, at the stage of transitioning to the protostellar ...phase, to harbor centrally concentrated dense (sub)structures that will seed the formation of a new star or the binary/multiple stellar systems. Characterizing this critical stage of evolution is key to our understanding of star formation. In this work, we report the detection of high-density (sub)structures on the thousand-astronomical-unit (au) scale in a sample of dense prestellar cores. Through our recent ALMA observations toward the Orion Planck Galactic Cold Clumps, we have found five extremely dense prestellar cores, which have centrally concentrated regions of ∼2000 au in size, and several 10
7
cm
−3
in average density. Masses of these centrally dense regions are in the range of 0.30 to 6.89
M
⊙
. For the first time, our higher resolution observations (0.8″ ∼ 320 au) further reveal that one of the cores shows clear signatures of fragmentation; such individual substructures/fragments have sizes of 800–1700 au, masses of 0.08 to 0.84
M
⊙
, densities of 2 − 8 × 10
7
cm
−3
, and separations of ∼1200 au. The substructures are massive enough (≳0.1
M
⊙
) to form young stellar objects and are likely examples of the earliest stage of stellar embryos that can lead to widely (∼1200 au) separated multiple systems.
ABSTRACT We present the results of a search for Very Low-Luminosity Objects (VeLLOs) in the Gould Belt (GB) clouds using infrared and sub-millimeter (sub-mm) data from 1.25 to 850 m and our (J = 1−0) ...observations. We modified the criteria by Dunham et al. to select the VeLLOs in the GB clouds, finding 95 VeLLO candidates, 79 of which are newly identified in this study. Out of 95 sources, 44 were detected in both sub-mm continuum and emission and were classified as Group A (the VeLLOs), and 51 sources detected in either sub-mm emission or emission were classified with Group B as candidate VeLLOs. We find that these VeLLOs and the candidates are forming in environments different from those of the likely VeLLOs. Seventy-eight sources are embedded within their molecular clouds, and thus are likely VeLLOs forming in a dense environment. The remaining 17 sources are located in low-level extinction regions ( ) connected to the clouds, and can be either background sources or candidate substellar objects forming in an isolated mode. The VeLLOs and the candidates are likely more luminous and their envelopes tend to be more massive in denser environments. The VeLLOs and the candidates are more populous in the clouds where more YSOs form, indicating that they form in a manner similar to that of normal YSOs. The bolometric luminosities and temperatures of the VeLLOs are compared to predictions of episodic accretion models, showing that the low luminosities for most VeLLOs can be well explained by their status in the quiescent phases of a cycle of episodic mass accretion.
Context.
LDN 1157 is one of several clouds that are situated in the cloud complex LDN 1147/1158. The cloud presents a coma-shaped morphology with a well-collimated bipolar outflow emanating from a ...Class 0 protostar, LDN 1157-mm, that resides deep inside the cloud.
Aims.
The main goals of this work are (a) mapping the intercloud magnetic field (ICMF) geometry of the region surrounding LDN 1157 to investigate its relationship with the cloud morphology, outflow direction, and core magnetic field (CMF) geometry inferred from the millimeter- and submillimeter polarization results from the literature, and (b) to investigate the kinematic structure of the cloud.
Methods.
We carried out optical (
R
-band) polarization observations of the stars projected on the cloud to map the parsec-scale magnetic field geometry. We made spectroscopic observations of the entire cloud in the
12
CO, C
18
O, and N
2
H
+
(
J
= 1–0) lines to investigate its kinematic structure.
Results.
We obtained a distance of 340 ± 3 pc to the LDN 1147/1158, complex based on the
Gaia
DR2 parallaxes and proper motion values of the three young stellar objects (YSOs) associated with the complex. A single filament of ~1.2 pc in length (traced by the
Filfinder
algorithm) and ~0.09 pc in width (estimated using the
Radfil
algorithm) is found to run throughout the coma-shaped cloud. Based on the relationships between the ICMF, CMF, filament orientations, outflow direction, and the hourglass morphology of the magnetic field, it is likely that the magnetic field played an important role in the star formation process in LDN 1157. LDN 1157-mm is embedded in one of the two high-density peaks detected using the
Clumpfind
algorithm. The two detected clumps lie on the filament and show a blue-red asymmetry in the
12
CO line. The C
18
O emission is well correlated with the filament and presents a coherent structure in velocity space. Combining the proper motions of the YSOs and the radial velocity of LDN 1147/1158 and an another complex, LDN 1172/1174, that is situated ~2° east of it, we found that the two complexes are moving collectively toward the Galactic plane. The filamentary morphology of the east-west segment of LDN 1157 may have formed as a result of mass lost by ablation through interaction of the moving cloud with the ambient interstellar medium.
Abstract
We present the results of a single-dish survey toward 95 very low luminosity objects (VeLLOs) in optically thick (HCN 1−0) and thin (N
2
H
+
1−0) lines performed for the purpose of ...understanding the physical processes of inward motions in the envelopes of the VeLLOs and characterizing their true nature. The normalized velocity differences (
δ
V
HCN
) between the peak velocities of the two lines were derived for 41 VeLLOs detected in both lines. The
δ
V
distribution of these VeLLOs is found to be significantly skewed to the blue, indicating the dominance of infalling motions in their envelopes. The infall speeds were derived for 15 infall candidates by using the HILL5 radiative transfer model. The speeds were in the range of 0.03−0.3 km s
−1
, with a median value of 0.16 km s
−1
, consistent with the gravitational freefall speeds from pressure-free envelopes. The mass infall rates calculated from the infall speeds are mostly of the order of 10
−6
M
⊙
yr
−1
, with a median value of (3.4 ± 1.5) × 10
−6
M
⊙
yr
−1
. These are found to be also consistent with the values predicted with the inside-out collapse model and show a fairly good correlation with the internal luminosities of the VeLLOs. This again indicates that the infall motions observed toward the VeLLOs are likely to be due to the gravitational infall motions in their envelopes. Our study suggests that most of the VeLLOs are potentially faint protostars, while two of the VeLLOs could possibly be proto−brown dwarf candidates.
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