R. Sahai

Jet-induced Star Formation in Centaurus A

The inner part of the northeast middle radio lobe of the radio galaxy Centaurus A is the site of complex interactions. This area contains a large H I cloud as well as filaments of ionized gas and associated blue knots, several of which exist along the northeastern edge of the radio-emitting zones. We observed the filaments and blue knots with the Hubble Space Telescope using WFPC2, and the ionized gas from the ground. Our sensitive, high angular resolution WFPC2 images reveal the presence of young stars, many concentrated in what appear to be OB associations, superimposed on a background sheet of older stars that is typical of the Cen A halo. The ages of the OB associations are estimated to be less than 15 Myr from a comparison of color-magnitude diagrams with those for the Large Magellanic Cloud star cluster NGC 2004, and younger stellar groups may be nearer regions of Hα emission. We discuss our data in the context of models for star formation stimulated by interactions between the radio jet and gas cloud.

HST observations of the protoplanetary nebula OH 231.8+4.2: The structure of the jets and shocks

We present high-resolution images obtained with the WFPC2, on board the HST, of the protoplanetary nebula (PPN) OH 231.8+4.2. H and NII line emission and scattered light in the continuum at 6750 and 7910 A were observed. We also discuss NIR NICMOS images from the HST archive. The images show with high accuracy the shape and excitation state of the shocks developed in the nebula. Our high-resolution images (and data from other works) allow a very detailed and quantitative description of the dierent nebular components and of the physical conditions in them. We interpret specic structures identied in our images using existing models of shock interaction. In the center of the nebula, there is a dense torus- or disk-like condensation continued by an hourglass-like structure, with relatively high densities (10 5 {10 6 cm 3 ) and temperatures (30 K). Inside this torus we have identied the location of the central star, from SiO maser observations. Two shock regions are detected from the optical line emission images, respectively in the north and south lobes. In both regions, a forward and a backward shock are identied. The densities of this hot gas vary between 40 and 250 cm 3 ,w ith the densest clumps being placed in the reverse shocks. The total mass of the shocked hot gas is210 3 M, both lobes showing similar masses in spite of their dierent extents. The relatively collimated jet that impinges on an originally slow shell, so producing the shocks, is identied from the scattered light images and in CO maps. This flow is signicantly denser and cooler than the shocked H regions. Its density decreases with the distance to the star, with typical values10 5 {10 4 cm 3 , and its temperature ranges between about 25 and 8 K. We explain the high H emission of the backward shock assuming that it propagates in a diuse gas component, entrained by the observed collimated flow and sharing its axial movement. The existence of shocks also in the collimated densest flow is suggested by the high abundance of some molecules like HCO + and its structure and kinematics in certain regions, but they are not seen in H emission, probably because of the absence of (well developed) hot components in this dense flow. We think that the exceptionally detailed and quantitative image derived for the wind interaction regions in OH 231.8+4.2 is a challenge to check and improve hydrodynamical models of wind interaction in PPNe.

The Age of the Sculptor Dwarf Spheroidal Galaxy from Imaging with WFPC21

ABSTRACT From images taken with the Hubble Space Telescopes WFPC2, we have obtained photometry of a field in the Sculptor dwarf spheroidal galaxy to 3 mag below the main‐sequence turnoff. We determine an age equal to that of the earliest globular clusters for the bulk of the stars in our field of view. We attempt to constrain the star formation history of the Sculptor dwarf by examining the main‐sequence luminosity function. The presence of a half‐dozen blue straggler candidates blueward of the turnoff points to a possibly complex star formation history. However, the contribution of any intermediate‐age population is difficult to measure conclusively, because of the uncertain origin of blue stragglers and the sparseness of the photometric sample.

Radio continuum monitoring of the extreme carbon star IRC+10216

We describe Very Large Array observations of the extreme carbon star IRC+10216 at 8.4, 14.9, and 22.5 GHz made over a two year period. We find possible variability correlated with the infrared phase and a cm- to sub-millimeter wavelength spectral index very close to 2. The variability, observed flux densities, and upper limit on the size are consistent with the emission arising from the stellar photosphere or a slightly larger radio photosphere.

Binarity and Accretion in AGB Stars: HST/STIS Observations of UV Flickering in Y Gem

Binarity is believed to dramatically affect the history and geometry of mass loss in AGB and post-AGB stars, but observational evidence of binarity is sorely lacking. As part of a project to search for hot binary companions to cool AGB stars using the GALEX archive, we discovered a late-M star, Y Gem, to be a source of strong and variable UV and X-ray emission. Here we report UV spectroscopic observations of Y Gem obtained with the Hubble Space Telescope that show strong flickering in the UV continuum on time-scales of ≲ 20 s, characteristic of an active accretion disk. Several UV lines with P-Cygni-type profiles from species such as Si IV and C IV are also observed, with emission and absorption features that are red- and blue-shifted by velocities of ~500 km s-1 from the systemic velocity. Our model for these (and previous) observations is that material from the primary star is gravitationally captured by a companion, producing a hot accretion disk. The latter powers a fast outflow that produces blue-shifted features due to absorption of UV continuum emited by the disk, whereas the red-shifted emission features arise in heated infalling material from the primary. The outflow velocities support a previous inference by Sahai et al. (2015) that Y Gems companion is a low-mass main-sequence star. Blackbody fitting of the UV continuum implies an accretion luminosity of about 13 L⊙, and thus a mass-accretion rate > 5 × 10-7M⊙ yr-1; we infer that Roche lobe overflow is the most likely binary accretion mode for Y Gem.

Understanding the immediate progenitors of planetary nebulae

Pre-Planetary Nebulae (PPNe) are believed to represent a relatively short, intermediate evolutionary phase in the evolution of AGB stars to Planetary Nebulae (PNe). Our unbiased, high-resolution imaging surveys with HST of young PNe and PPNe show very strong morphological similarities between these classes, enabling us to extend our morphological scheme for PPN classification to young PNe, preserving virtually all of the primary and secondary descriptors, and adding a few new ones. These morphological surveys show that the primary shaping of PNe begins during the PPNe and/or late-AGB phase, and the key to understanding the shaping process lies in the study of these PNe progenitors. Here we present results from two recent studies of PNe progenitors: (i) Hα emission with very broad wings and P-Cygni profiles as probes of the region where the fast post-AGB outflows that do the shaping are most likely launched from, and (ii) equatorial waists with large-sized dust grains and large masses.

Normal, Nascent and Stalled Pre-Planetary Nebulae

The formation and shaping of planetary nebulae (PNe) is probably the most exciting, yet least understood issue in the late evolution of (1–8)M stars. PNe evolve from the envelopes of AGB stars through the supposedly very short ( 10 yr) pre-PN (or PPN) phase (Balick & Frank 2002). In 1998, in a radical departure from the then long-standing theoretical paradigm for PN formation, Sahai & Trauger (1998) proposed that as most stars evolve off the AGB, they drive collimated fast winds that sweep up and shock the AGB circumstellar envelope, producing the observed dramatic changes in circumstellar geometry and kinematics from the AGB to the PN phase. Pre-Planetary Nebulae (PPNs) have traditionally been understood as rare objects that represent a transitory phase in the evolution of AGB stars to PNs. In recent years, mainly due to high-resolution imaging surveys with HST, it has become possible to start studying the detailed physical properties for a statistically significant number of these objects. Here we provide a brief report of results from our several large surveys of PPNs with HST (and supporting ground-based observations).