With the advent of high-resolution high-sensitivity observations, spiral patterns have been revealed around several asymptotic giant branch (AGB) stars. Such patterns can provide possible evidence for the existence of central binary stars embedded in
outflowing circumstellar envelopes. Here, we suggest the viability of explaining the previously observed incomplete ring-like patterns with the spiral-shell structure due to the motion of (unknown) binary components viewed at an inclination with respect to the orbital plane. We describe a method of extracting such spiral-shells from an incomplete ring-like pattern to place constraints on the characteristics of the central binary stars. The use of gas kinematics is essential in facilitating a detailed modeling for the three-dimensional structure of the circumstellar pattern. We show that a hydrodynamic radiative transfer model can reproduce the structure of the HC3N molecular line emission of the extreme carbon star, CIT 6. This method can be applied to other sources in the AGB phase and to the outer ring-like patterns of pre-planetary nebulae for probing the existence of embedded binary stars, which are highly anticipated with future observations using the Atacama Large Millimeter/submillimeter Array.
Clouds of high infrared extinction are promising sites of massive star/cluster formation. A large number of cloud cores discovered in recent years allows investigation of possible evolutionary sequence among cores in early phases. We have conducted a
survey of deuterium fractionation toward 15 dense cores in various evolutionary stages, from high-mass starless cores to ultracompact Hii regions, in the massive star-forming clouds of high extinction, G34.43+0.24, IRAS 18151-1208, and IRAS 18223-1243, with the Submillimeter Telescope (SMT). Spectra of N2H+ (3 - 2), N2D+ (3 - 2), and C18O (2 - 1) were observed to derive the deuterium fractionation of N2H+, Dfrac equiv N(N2D+)/N(N2H+), as well as the CO depletion factor for every selected core. Our results show a decreasing trend in Dfrac with both gas temperature and linewidth. Since colder and quiescent gas is likely to be associated with less evolved cores, larger Dfrac appears to correlate with early phases of core evolution. Such decreasing trend resembles the behavior of Dfrac in the low-mass protostellar cores and is consistent with several earlier studies in high-mass protostellar cores. We also find a moderate increasing trend of Dfrac with the CO depletion factor, suggesting that sublimation of ice mantles alters the competition in the chemical reactions and reduces Dfrac. Our findings suggest a general chemical behavior of deuterated species in both low- and high-mass proto-stellar candidates at early stages. In addition, upper limits to the ionization degree are estimated to be within 2 times 10^-7 and 5 times 10^-6. The four quiescent cores have marginal field-neutral coupling and perhaps favor turbulent cooling flows.
The protostellar jet driven by L1448C was observed in the SiO J=8-7 and CO J=3-2 lines and 350 GHz dust continuum at ~1 resolution with the Submillimeter Array (SMA). A narrow jet from the northern source L1448C(N) was observed in the SiO and the hig
h-velocity CO. The jet consists of a chain of emission knots with an inter-knot spacing of ~2 (500 AU) and a semi-periodic velocity variation. The innermost pair of knots, which are significant in the SiO map but barely seen in the CO, are located at ~1 (250 AU) from the central source, L1448C(N). Since the dynamical time scale for the innermost pair is only ~10 yr, SiO may have been formed in the protostellar wind through the gas-phase reaction, or been formed on the dust grain and directly released into the gas phase by means of shocks. It is found that the jet is extremely active with a mechanical luminosity of ~7 L_sun, which is comparable to the bolometric luminosity of the central source (7.5 L_sun). The mass accretion rate onto the protostar derived from the mass-loss rate is ~10^{-5} M_sun/yr. Such a high mass accretion rate suggests that the mass and the age of the central star are 0.03-0.09 M_sun and (4-12)x10^3 yr, respectively, implying that the central star is in the very early stage of protostellar evolution. The low-velocity CO emission delineates two V-shaped shells with a common apex at L1448C(N). The kinematics of these shells are reproduced by the model of a wide opening angle wind. The co-existence of the highly-collimated jets and the wide-opening angle shells can be explained by the unified X-wind model in which highly-collimated jet components correspond to the on-axis density enhancement of the wide-opening angle wind. The CO $J$=3--2 map also revealed the second outflow driven by the southern source L1448C(S) located at ~8.3 (2000 AU) from L1448C(N).
We have observed the J=3-2 transition of N2H+ and N2D+ to investigate the trend of deuterium fractionation with evolutionary stage in three selected regions in the Infrared Dark Cloud (IRDC) G28.34+0.06 with the Submillimeter Telescope (SMT) and the
Submillimeter Array (SMA). A comprehensible enhancement of roughly 3 orders of magnitude in deuterium fractionation over the local interstellar D/H ratio is observed in all sources. In particular, our sample of massive star-forming cores in G28.34+0.06 shows a moderate decreasing trend over a factor of 3 in the N(N2D+)/N(N2H+) ratio with evolutionary stage, a behavior resembling what previously found in low-mass protostellar cores. This suggests a possible extension for the use of the N(N2D+)/N(N2H+) ratio as an evolutionary tracer to high-mass protostellar candidates. In the most evolved core, MM1, the N2H+ (3-2) emission appears to avoid the warm region traced by dust continuum emission and emission of 13CO sublimated from grain mantles, indicating an instant release of gas-phase CO. The majority of the N2H+ and N2D+ emission is associated with extended structures larger than 8 (~ 0.2 pc).
