Stellar Evolution

Thousands of extrasolar planets (“exoplanets”) have been discovered over the past two decades. Astronomers seeking to understand the astonishing variety of planetary masses and orbital separations that characterize these myriad exoplanet systems, as well as the earliest evolution of our own so- lar system, must carefully study exoplanet birthplaces: dusty, molecule-rich protoplanetary disks orbiting young stars. LAMA astronomers are exploiting the recently commissioned Atacama Large Millimeter Array (ALMA) radio interferometer as well as the latest generation of adaptive optics (AO) cameras on the worlds largest (8-meter-class) optical/infrared telescopes to make advances in the study of protoplanetary disks.

A prime example is the “library” of subarcsecond-resolution ALMA molec- ular emission images of the nearby, young, close binary system V4046 Sagitarii that was recently assembled, analyzed, and published by LAMA’s Kastner, RIT AST graduate student Annie Dickson-Vandervelde, and collaborators. V4046 Sgr is a binary T Tauri system of age ∼20 Myr that lies just 72.4 pc from Earth. The ALMA observations were obtained in the 1.1–1.4 mm wavelength range with antenna configurations involving maximum baselines of several hundred meters, yielding subarcsecond-resolution images in more than a dozen molecular species and isotopologues (Fig. 1). This ALMA image library of V4046 Sgr hence elucidates, on linear size scales of ∼30–40 au, the chemical structure of the evolved, protoplanetary disk orbiting the close (2.4 day period) binary system. This study of the “molecular anatomy” of the circumbinary disk orbiting V4046 Sgr should serve as motivation for additional subarcsecond ALMA molecular line imaging surveys of nearby, evolved protoplanetary disks aimed at addressing major uncertainties in protoplanetary disk physical and chemical structure and molecular production pathways.

In another new ALMA-based paper on V4046 Sgr, LAMA postdoc Ruiz-Rodriguez, Kastner, and team present and analyze the highest-resolution continuum and scattered-light images obtained to date of this circumbinary disk. Ruiz-Rodriguez et al. observed the disk with ALMA at 870 μm (Band 7) during Cycle 4, and analyzed these data in conjunction with archival H band polarimetric images taken with SPHERE/IRDIS on the ESO Very Large Telescope. They investigated whether the ring structures detected in these images can be accounted for by models that combine two-dimensional two-fluid (gas + particle) hydrodynamical calculations with three-dimensional Monte Carlo radiative transfer simulations to simultaneously model the potential observational signatures of protoplanet-induced gaps at millimeter and near-infrared wavelengths. Ruiz-Rodriguez et al. find that a single planet with a mass in the range between 0.3 and 1.5 MJ orbiting at 20 au from the central star can well reproduce the combination of a deep gap in scattered light and a surrounding bright 870 μm ring. These results represent perhaps the best present constraints on the mass of a putative (as-yet undetected) circumbinary protoplanet.

LAMA astronomers are also developing strategies to search for and characterize additional examples of nearby, young, low-mass stars by combining data from NASA’s high-energy archives, such as Galex UV photometry and X-ray data from ROSAT, Chandra, and XMM, with Gaia astrometric and photometric data. In a recent study, we selected nearly 400 candidate nearby, young, late-type stars in the approximate mass range 0.5–1.0 M⊙ from the Gaia Data Release 1 (DR1) TGAS catalog on the basis of (a) D < 125 pc, (b) Galex UV detection, and (c) isochronal age ≤80 Myr. Approximately 10% of these candidates lie within 50 pc of Earth and, hence, potentially represent excellent targets for direct-imaging searches for young, self-luminous planets. We have performed a complete spectroscopic and kinematic analysis of this sample, folding in data from Gaia Data Release 2 (DR2), 2MASS/WISE data, ROSAT X-ray data, and optical/IR data from Vizier catalogs. This analysis has established that only a small percentage (<10%) of stars among this (kinematically unbiased) sample can be confidently associated with established nearby, young moving groups (NYMGs). The majority display anomalous kinematics, relative to the known NYMGs. In addition to their non-young-star-like kinematics, the majority of the UV-selected, isochronally young field stars within 50 pc appear surprisingly X-ray faint. These stars may hence represent a previously unrecognized population of young stars that has recently mixed into the older field star population.

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Hubble Space Telescope image of the Helix planetary nebula, with positions observed by the IRAM 30 meter radio telescope indicated. The right-hand panels show molecular line spectra collected at two representative positions. Figure adapted from Bublitz et al. (in preparation).

Planetary nebulae form after Sun-like stars go through a phase of extreme mass loss at the end of their life. These objects lose a majority of their mass (primarily hydrogen and helium, but also carbon, nitrogen, oxygen, and occasionally heavier elements) as the material is pushed outward from the surface of the dying star by radiation pressure and shocks until, eventually, only a hot core remains. As the hotter layers of the stellar core are exposed, they produce high-energy radiation. UV and X-rays from the central star irradiate the expanding shells of ejected gas. The atoms and molecules in the gas re-emit this energy across longer wavelengths like radio and infrared, which we can detect. The planetary nebula has now formed and through this process, Sun-like stars generate crucial atoms (such as C and O) that are ejected into space and might eventually be incorporated into future generations of stars or even planets and potential life on those planets.

Certain planetary nebulae contain shells, filaments, or globules of cold gas and dust whose heating and chemistry are likely driven by UV and X-ray emission from their central stars and from wind-collision-generated shocks. In 2018, LAMA Ph.D. student Jesse Bublitz, Kastner, and their international team of planetary nebula experts completed analysis of a survey of molecular line emission in the mm wavelength regime range from nine nearby nebulae using the 30-meter radio telescope operated by Europe's Institut de Radioastronomie Millimétrique (IRAM). Rotational transitions of thirteen molecules, including CO, its isotopologues, and chemically important trace species such as HCN, HNC, CN, and HCO+, were observed and the results compared with and augmented by previous studies of molecular gas in planetary nebulae. Emission lines of the aforementioned molecules were detected in most objects, and one or more of these represent new detections for five of the planetary nebulae studied. Among other results, the Bublitz et al. survey and analysis reveals that the abundance of HCN relative to its molecular isomer HNC within a given planetary nebula is critically dependent on the power of UV radiation from the nebula's central star. This surprisingly robust correlation between HNC/HCN ratio and central star UV radiation demonstrates the potential of molecular emission line studies of PNe for improving our understanding of the role that high-energy radiation plays in the heating and chemistry of molecular gas in interstellar space. As 2018 drew to a close, the results of the Bublitz team's IRAM 30-meter telescope survey of planetary nebulae had been submitted for publication in the European journal Astronomy & Astrophysics.

Bublitz and team have followed up on their 30-meter telescope survey by scrutinizing the Helix planetary nebula more closely with the IRAM 30-meter.  The new observations demonstrate that the Helix -- a particularly nearby, large, molecule-rich planetary nebula that is easily visible in a small telescope at a dark site -- shows a significant decline in its HNC/HCN ratio as one approaches the central star. These results provide "ground truth" for the hypothesis that the relative abundances of HNC and HCN are determined in large part by central star UV radiation.