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Register a new userThe Circumstellar Environment of the Early B Protostar G192.16-3.84 and the Discovery of a Low-Mass, Protostellar Core
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D.S. Shepherd
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We have observed the massive star forming region associated with the early B protostar G192.16-3.84 in NH3(1,1), 22.2 GHz water masers, 1.3 cm continuum emission, and at 850 microns. The dense gas associated with G192.16 is clumpy, optically thin, and has a mass of 0.9 Msun. The ammonia core is gravitationally unstable which may signal that the outflow phase of this system is coming to an end. Water masers trace an ionized jet 0.8 (1600 AU at a distance of 2 kpc) north of G192.16. Masers are also located within 500 AU of G192.16, their velocity distribution is consistent with but does not strongly support the interpretation that the maser emission arises in a 1000 AU rotating disk centered on G192.16. Roughly 30 south of G192.16 (0.3 pc) is a compact, optically thick (optical depth = 1.2) ammonia core (called G192 S3) with an estimated mass of 2.6 Msun. Based on the presence of 850 micron and 1.2 mm continuum emission, G192 S3 probably harbors a very young, low-mass protostar or proto-cluster. The dense gas in the G192 S3 core is likely to be gravitationally bound and may represent the next site of star formation in this region.
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Previous observations have revealed an accretion disk and outflow motion in high-mass star-forming region G192.16-3.84. While collapse have not been reported before. We present here molecular line and continuum observations toward massive core G192.16-3.84 with the Submillimeter Array. C$^{18}$O(2-1) and HCO$^{+}$(3-2) lines show pronounced blue profiles, indicating gas infalling in this region. This is the first time that the infall motion has been reported in G192.16-3.84 core. Two-layer model fitting gave infall velocities of 2.0$pm$0.2 and 2.8$pm$0.1 km s$^{-1}$. Assuming that the cloud core follows a power-law density profile ($rho$$propto$$r^{1.5}$), the corresponding mass infall rates are (4.7$pm$1.7)$times10^{-3}$ and (6.6$pm$2.1)$times10^{-3}$ M$_{sun}$ yr$^{-1}$ for C$^{18}$O(2-1) and HCO$^{+}$(3-2), respectively. The derived infall rates are in agreement with the turbulent core model and those in other high-mass star-forming regions, suggesting that high accretion rate is a general requirement to form a massive star.
We present the results of astrometic observations of H2O masers associated with the star forming region G192.16-3.84 with the VLBI Exploration of Radio Astrometry (VERA). The H2O masers seem to be associated with two young stellar objects (YSOs) separated by sim1200 AU as reported in previous observations. In the present observations, we successfully detected an annual parallax of 0.66 pm 0.04 mas for the H2 O masers, which corresponds to a distance to G192.16-3.84 of D = 1.52 pm 0.08 kpc from the Sun. The determined distance is shorter than the estimated kinematic distance. Using the annual parallax distance and the estimated parameters of the millimeter continuum emission, we estimate the mass of the disk plus circumstellar cloud in the southern young stellar object to be 10.0+4.3Mcdot. We also estimate the galactocentric distance and the peculiar motion -3.6 of G192.16-3.84, relative to a circular Galactic rotation: Rstar = 9.99 pm 0.08 kpc, Zstar = -0.10 pm 0.01 kpc, and (Ustar,Vstar,Wstar)=(-2.8pm1.0,-10.5pm0.3,4.9pm2.7)[kms-1]respectively. The peculiar motion of G192.16-3.84 is within that typically found in recent VLBI astrometric results. The angular distribution and three-dimensional velocity field of H2O maser features associated with the northern YSO indicate the existence of a bipolar outflow with a major axis along the northeast-southwest direction.
We observe 84 candidate young high-mass sources in the rare isotopologues C17O and C18O to investigate whether there is evidence for depletion (freeze-out) towards these objects. Observations of the J=2-1 transitions of C18O and C17O are used to derive the column densities of gas towards the sources and these are compared with those derived from submillimetre continuum observations. The derived fractional abundance suggests that the CO species show a range of degrees of depletion towards the objects. We then use the radiative transfer code RATRAN to model a selection of the sources to confirm that the spread of abundances is not a result of assumptions made when calculating the column densities. We find a range of abundances of C17O that cannot be accounted for by global variations in either the temperature or dust properties and so must reflect source to source variations. The most likely explanation is that different sources show different degrees of depletion of the CO. Comparison of the C17O linewidths of our sources with those of CS presented by other authors reveal a division of the sources into two groups. Sources with a CS linewidth >3 km/s have low abundances of C17O while sources with narrower CS lines have typically higher C17O abundances. We suggest that this represents an evolutionary trend. Depletion towards these objects shows that the gas remains cold and dense for long enough for the trace species to deplete. The range of depletion measured suggests that these objects have lifetimes of 2-4x10^5 years.
EC53 is an embedded protostar with quasi-periodic emission in the near-IR and sub-mm. We use ALMA high-resolution observations of continuum and molecular line emission to describe the circumstellar environment of EC 53. The continuum image reveals a disk with a flux that suggests a mass of 0.075 Msun, much less than the estimated mass in the envelope, and an in-band spectral index that indicates grain growth to centimeter sizes. Molecular lines trace the outflow cavity walls, infalling and rotating envelope, and/or the Keplerian disk. The rotation profile of the C17O 3-2 line emission cannot isolate the Keplerian motion clearly although the lower limit of the protostellar mass can be calculated as 0.3 +- 0.1 Msun if the Keplerian motion is adopted. The weak CH3OH emission, which is anti-correlated with the HCO+ 4-3 line emission, indicates that the water snow line is more extended than what expected from the current luminosity, attesting to bygone outburst events. The extended snow line may persist for longer at the disk surface because the lower density increases the freeze-out timescale of methanol and water.
We report the detection of complex molecules (HCOOCH_3, HCOOH and CH_3CN), signposts of a hot core like region, toward the low mass, Class 0 source NGC1333-IRAS4A. This is the second low mass protostar where such complex molecules have been searched for and reported, the other source being IRAS16293-2422. It is therefore likely that compact (few tens of AUs) regions of dense and warm gas, where the chemistry is dominated by the evaporation of grain mantles, and where complex molecules are found, are common in low mass Class 0 sources.Given that the chemical formation timescale is much shorter than the gas hot core crossing time, it is not clear whether the reported complex molecules are formed on the grain surfaces (first generation molecules) or in the warm gas by reactions involving the evaporated mantle constituents (second generation molecules). We do not find evidence for large differences in the molecular abundances, normalized to the formaldehyde abundance, between the two solar type protostars, suggesting perhaps a common origin.
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