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We used the Extended Submillimeter Array (eSMA) in its most extended configuration to investigate the innermost (within a radius of 290 R* from the star) circumstellar envelope (CSE) of IRC+10216. We imaged the CSE using HCN and other molecular lines with a beam size of 0.22 x 0.46, deeply into the very inner edge (15 R*) of the envelope where the expansion velocity is only 3 km/s. The excitation mechanism of hot HCN and KCl maser lines is discussed. HCN maser components are spatially resolved for the first time on an astronomical object. We identified two discrete regions in the envelope: a region with a radius of . 15 R*, where molecular species have just formed and the gas has begun to be accelerated (region I) and a shell region (region II) with a radius of 23 R* and a thickness of 15 R*, whose expansion velocity has reached up to 13 km/s, nearly the terminal velocity of 15 km/s. The Si$^{34}$S line detected in region I shows a large expansion velocity of 16 km/s due to strong wing components, indicating that the emission may arise from a shock region in the innermost envelope. In region II, the P.A. of the most copious mass loss direction was found to be 120 +/- 10 degrees, which may correspond to the equatorial direction of the star. Region II contains a torus-like feature. These two regions may have emerged due to significant differences in the size distributions of the dust particles in the two regions.
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We analyze the properties of the innermost narrow line region in a sample of low-luminosity AGN. We select 33 LINERs (bona fide AGN) and Seyfert galaxies from the optical spectroscopic Palomar survey observed by HST/STIS. We find that in LINERs the [NII] and [OI] lines are broader than the [SII] line and that the [NII]/[SII] flux ratio increases when moving from ground-based to HST spectra. This effect is more pronounced considering the wings of the lines. Our interpretation is that, as a result of superior HST spatial resolution, we isolate a compact region of dense ionized gas in LINERs, located at a typical distance of about 3 pc and with a gas density of about 10$^4$-10$^5$ cm$^{-3}$, which we identify with the outer portion of the intermediate line region (ILR). Instead, we do not observe these kinds of effects in Seyferts; this may be the result of a stronger dilution from the NLR emission, since the HST slit maps a larger region in these sources. Alternatively, we argue that the innermost, higher density component of the ILR is only present in Seyferts, while it is truncated at larger radii because of the presence of the circumnuclear torus. The ILR is only visible in its entirety in LINERs because the obscuring torus is not present in these sources.
During their late pulsating phase, AGB stars expel most of their mass in the form of massive dusty envelopes, an event that largely controls the composition of interstellar matter. The envelopes, however, are distant and opaque to visible and NIR radiation: Their structure remains poorly known and the mass-loss process poorly understood. Millimeter-wave interferometry is the optimal investigative tool for this purpose. The circumstellar envelope IRC +10 216 and its central star, the C-rich TP-AGB star closest to the Sun, are the best objects for such an investigation. Two years ago, we reported on IRAM 30-m telescope CO(2-1) line emission observations in that envelope (HPBW 11). We now report much higher angular resolution observations of CO(2-1), CO(1-0), CN(2-1) and C$_4$H(24-23) made with the SMA, PdB and ALMA interferometers (with synthesized half-power beamwidths of 3, 1 and 0.3, respectively). Although the envelope appears more intricate at high resolution, its prevailing structure remains a pattern of thin, nearly concentric shells. Outside the small (r<0.3) dust formation zone, the gas appears to expand radially at a constant velocity, 14.5 km/s, with small turbulent motions. Based on that property, we have reconstructed the 3-D structure of the outer envelope and have derived the gas temperature and density radial profiles in the inner (r<25) envelope. The over-dense shells have spherical or slightly oblate shapes and typically extend over a few steradians, implying isotropic mass loss. The regular spacing of shells in the outer envelope supports the model of a binary star system with a period of 700 years and a near face-on elliptical orbit. The companion fly-by triggers enhanced episodes of mass loss near periastron. The densification of the shell pattern observed in the central part of the envelope suggests a more complex scenario for the last few thousand years.
The circumstellar envelope of the hypergiant star IRC+10420 has been traced as far out in SiO J=2-1 as in CO J = 1-0 and CO J = 2-1, in dramatic contrast with the centrally condensed (thermal) SiO- but extended CO-emitting envelopes of giant and supergiant stars. Here, we present an observation of the circumstellar envelope in SiO J=1-0 that, when combined with the previous observation in {sioii}, provide more stringent constraints on the density of the SiO-emitting gas than hitherto possible. The emission in SiO peaks at a radius of $sim$2arcsec whereas that in SiO J=2-1 emission peaks at a smaller radius of $sim$1arcsec, giving rise to their ring-like appearances. The ratio in brightness temperature between SiO J=1-0 and SiO J=2-1 decreases from a value well above unity at the innermost measurable radius to about unity at radius of $sim$2arcsec, beyond which this ratio remains approximately constant. Dividing the envelope into three zones as in models for the CO J = 1-0 and CO J = 2-1 emission, we show that the density of the SiO-emitting gas is comparable with that of the CO-emitting gas in the inner zone, but at least an order of magnitude higher by comparison in both the middle and outer zones. The SiO-emitting gas therefore originates from dense clumps, likely associated with the dust clumps seen in scattered optical light, surrounded by more diffuse CO-emitting interclump gas. We suggest that SiO molecules are released from dust grains due to shock interactions between the dense SiO-emitting clumps and the diffuse CO-emitting interclump gas.
Context: The protostellar envelopes, outflow and large-scale chemistry of Class~0 and Class~I objects have been well-studied, but while previous works have hinted at or found a few Keplerian disks at the Class~0 stage, it remains to be seen if their presence in this early stage is the norm. Likewise, while complex organics have been detected toward some Class~0 objects, their distribution is unknown as they could reside in the hottest parts of the envelope, in the emerging disk itself or in other components of the protostellar system, such as shocked regions related to outflows. Aims: In this work, we aim to address two related issues regarding protostars: when rotationally supported disks form around deeply embedded protostars and where complex organic molecules reside in such objects. Methods: We observed the deeply embedded protostar, L483, using Atacama Large Millimeter/submillimeter Array (ALMA) Band~7 data from Cycles~1 and 3 with a high angular resolution down to $sim$~0.1$^{primeprime}$ (20~au) scales. Results: We find that the kinematics of CS~$J=7$--$6$ and H$^{13}$CN~$J=4$--$3$ are best fitted by the velocity profile from infall under conservation of angular momentum and not by a Keplerian profile. The spatial extents of the observed complex organics are consistent with an estimated ice sublimation radius of the envelope at $sim$~50~au, suggesting that the complex organics exist in the hot corino of L483. Conclusions: We find that L483 does not harbor a Keplerian disk down to at least $15$~au in radius. Instead, the innermost regions of L483 are undergoing a rotating collapse. This result highlights that some Class~0 objects contain only very small disks, or none at all, with the complex organic chemistry taking place on scales inside the hot corino of the envelope, in a region larger than the emerging disk.
The massive young stellar object S255IR NIRS3 embedded in the star forming core SMA1 has been recently observed with a luminosity burst, which is conjectured as a disc-mediated variable accretion event. In this context, it is imperative to characterize the gas properties around the massive young stellar object. With this in mind, we carried out high angular resolution observations with the Atacama Large Millimeter and submillimeter Array and imaged the 900 $mu m$ dust continuum and the CH$_3$CN $J$=19$-$18 $K$=0$-$10 transitions of S255IR SMA1. The integrated CH$_3$CN emission exhibits an elongated feature with an extent of 1800 au in the northwest-southeast direction at a position angle of 165 degree, which is nearly perpendicular to the bipolar outflow. We confirm the presence of dense (a few $times 10^{9}$ cm$^{-3}$) and hot ($sim$ 400 K) gas immediately surrounding the central protostar. The CH$_3$CN emission features a velocity gradient along the elongated ridge and by modelling the gas kinematics based on features in the position-velocity diagram, we infer that the gas is best described by a flattened rotating infalling envelope (or pseudo-disc). A mass infall rate of a few $times$ 10$^{-4}$ solar-mass per year is derived. If there exists a putative Keplerian disc directly involved in the mass accretion onto the star and jet/outflow launching, it is likely smaller than 125 au and unresolved by our observations. We show qualitative resemblances between the gas properties (such as density and kinematics) in 255IR SMA1 inferred from our observations and those in a numerical simulation particularly tailored for studying the burst mode of massive star formation.
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