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Register a new userThe nature of very low luminosity objects (VeLLOs)
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Eduard I. Vorobyov
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The nature of very low luminosity objects with the internal luminosity L_obj <= 0.1 Lsun is investigated by means of numerical modeling coupling the core collapse simulations with the stellar evolution calculations. The gravitational collapse of a large sample of model cores in the mass range 0.1-2.0 Msun is investigated. Numerical simulations were started at the pre-stellar phase and terminated at the end of the embedded phase when 90% of the initial core mass had been accreted onto the forming protostar plus disk system. The disk formation and evolution was studied using numerical hydrodynamics simulations, while the formation and evolution of the central star was calculated using a stellar evolution code. Three scenarios for mass accretion from the disk onto the star were considered: hybrid accretion in which a fraction of accreted energy absorbed by the protostar depends on the accretion rate, hot accretion wherein a fraction of accreted energy is constant, and cold accretion wherein all accretion energy is radiated away. Our conclusions on the nature of VeLLOs depend crucially on the character of protostellar accretion. In the hybrid accretion scenario, most VeLLOs (90.6%) are expected to be the first hydrostatic cores (FHSCs) and only a small fraction (9.4%) are true protostars. In the hot accretion scenario, all VeLLOs are FHSCs due to overly high photospheric luminosity of protostars. In the cold accretion scenario, on the contrary, the majority of VeLLOs belong to the Class I phase of stellar evolution. The reason is that the stellar photospheric luminosity, which sets the floor for the total internal luminosity of a young star, is lower in cold accretion, thus enabling more VeLLOs in the protostellar stage. VeLLOs are relatively rare objects occupying 7%-11% of the total duration of the embedded phase and their masses do not exceed 0.3 Msun. (abridged).
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We present a systematic single-dish search for molecular outflows toward a sample of 9 candidate low-luminosity protostars and 30 candidate Very Low Luminosity Objects (VeLLOs; L_int < 0.1 L_sun). The sources are identified using data from the Spitzer Space Telescope catalogued by Dunham et al. toward nearby (D < 400 pc) star forming regions. Each object was observed in 12CO and 13CO J = 2-1 simultaneously using the sideband separating ALMA Band-6 prototype receiver on the Heinrich Hertz Telescope at 30 arcsecond resolution. Using 5-point grid maps we identify five new potential outflow candidates and make on-the-fly maps of the regions surrounding sources in the dense cores B59, L1148, L1228, and L1165. Of these new outflow candidates, only the map of B59 shows a candidate blue outflow lobe associated with a source in our survey. We also present larger and more sensitive maps of the previously detected L673-7 and the L1251-A IRS4 outflows and analyze their properties in comparison to other outflows from VeLLOs. The accretion luminosities derived from the outflow properties of the VeLLOs with detected CO outflows are higher than the observed internal luminosity of the protostars, indicating that these sources likely had higher accretion rates in the past. The known L1251-A IRS3 outflow is detected but not remapped. We do not detect clear, unconfused signatures of red and blue molecular wings toward the other 31 sources in the survey indicating that large-scale, distinct outflows are rare toward this sample of candidate protostars. Several potential outflows are confused with kinematic structure in the surrounding core and cloud. Interferometric imaging is needed to disentangle large-scale molecular cloud kinematics from these potentially weak protostellar outflows.
[Abridged] We carried out optical polarimetry of five dense cores, (IRAM 04191, L1521F, L328, L673-7, and L1014) which are found to harbour VeLLO. This study was conducted mainly to understand the role played by the magnetic field in the formation of very low and substellar mass range objects using optical polarisation. The angular offsets between the envelope magnetic field direction (inferred from optical polarisation measurements) and the outflow position angles from the VeLLOs in IRAM 04191, L1521F, L328, L673-7, and L1014 are found to be 84$^circ$, 53$^circ$, 24$^circ$, 08$^circ$, and 15$^circ$, respectively. The mean value of the offsets for all the five clouds is $sim37^circ$. If we exclude IRAM 04191, the mean value reduces to become $sim25^circ$. In IRAM 04191, the offset between the projected envelope and the inner magnetic field (inferred from the submillimetre data from SCUPOL) is found to be $sim68^circ$. The inner magnetic field, however, is found to be nearly aligned with the projected position angles of the minor axis, the rotation axis of the cloud, and the outflow from the IRAM 04191-IRS. We discuss a possible explanation for the nearly perpendicular orientation between the envelope and core scale magnetic fields in IRAM04191. The angular offset between the envelope magnetic field direction and the minor axis of IRAM 04191, L1521F, L673-7, and L1014 are 82$^circ$, 60$^circ$, 47$^circ$, and 55$^circ$, respectively. The mean value of the offsets between the envelope magnetic field and the minor axis position angles for the four cores is found to be $sim60^circ$. The results obtained from our study on the limited sample of five cores with VeLLOs show that the outflows in three of them tend to nearly align with the envelope magnetic field.
We present the results of a single dish survey toward 95 VeLLOs in optically thick (HCN 1-0) and thin ($rm N_2H^+$ 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 ($delta V_{HCN}$) between the peak velocities of the two lines were derived for 41 VeLLOs detected in both lines. The $delta V$ 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 $rm km~s^{-1}$ to 0.3 $rm km~s^{-1}$, with a median value of 0.16 $rm km~s^{-1}$, being consistent with the gravitational free-fall speeds from pressure-free envelopes. The mass infall rates calculated from the infall speeds are mostly of the order of $10^{-6} M_{odot}~yr^{-1}$ with a median value of $rm 3.4 pm 1.5 times 10^{-6} M_{odot}~yr^{-1}$. 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.
We present a survey for water maser emission toward a sample of 44 low-luminosity young objects, comprising (proto-)brown dwarfs, first hydrostatic cores (FHCs), and other young stellar objects (YSOs) with bolometric luminosities lower than 0.4 L$_odot$. Water maser emission is a good tracer of energetic processes, such as mass-loss and/or accretion, and is a useful tool to study this processes with very high angular resolution. This type of emission has been confirmed in objects with L$_{rm bol}ge 1$ L$_odot$. Objects with lower luminosities also undergo mass-loss and accretion, and thus, are prospective sites of maser emission. Our sensitive single-dish observations provided a single detection when pointing toward the FHC L1448 IRS 2E. However, follow-up interferometric observations showed water maser emission associated with the nearby YSO L1448 IRS 2 { (a Class 0 protostar of L$_{rm bol}simeq 3.6-5.3$ L$_odot$)}, and did not find any emission toward L1448 IRS 2E. The upper limits for water maser emission determined by our observations are one order of magnitude lower than expected from the correlation between water maser luminosities and bolometric luminosities found for YSOs. This suggests that this correlation does not hold at the lower end of the (sub)stellar mass spectrum. Possible reasons are that the slope of this correlation is steeper at L$_{rm bol}le 1$ L$_odot$, or that there is an absolute luminosity threshold below which water maser emission cannot be produced. Alternatively, if the correlation still stands at low luminosity, the detection rates of masers would be significantly lower than the values obtained in higher-luminosity Class 0 protostars.
Very low-mass stars are known to have jets and outflows, which is indicative of a scaled-down version of low-mass star formation. However, only very few outflows in very low-mass sources are well characterized. We characterize the bipolar molecular outflow of the very low-mass star Par-Lup3-4, a 0.12 M$_{odot}$ object known to power an optical jet. We observed Par-Lup3-4 with ALMA in Bands 6 and 7, detecting both the continuum and CO molecular gas. In particular, we studied three main emission lines: CO(2-1), CO(3-2), and $^{13}$CO(3-2). Our observations reveal for the first time the base of a bipolar molecular outflow in a very low-mass star, as well as a stream of material moving perpendicular to the primary outflow of this source. The primary outflow morphology is consistent with the previously determined jet orientation and disk inclination. The outflow mass is $9.5times10^{-7}mathrm{M}_{odot}$ , with an outflow rate of $4.3times10^{-9}mathrm{M}_{odot}mathrm{yr}^{-1}$ A new fitting to the spectral energy distribution suggests that Par-Lup3-4 may be a binary system. We have characterized Par-Lup3-4 in detail, and its properties are consistent with those reported in other very low-mass sources. This source provides further evidence that very low-mass sources form as a scaled-down version of low-mass stars.
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