Galactic Astrophysics

Young stars and their planet-forming disks 

Circumstellar disks around recently formed stars serve both as the source of material for the young star itself and as the formation sites of young planets orbiting such a star. Observations that can establish the physical conditions and evolution of the gas within circumstellar disks around young stars are essential if astronomers are to understand: the so-called accretion process that determines a star’s initial mass and, hence, governs the star’s subsequent evolution; the processes involved in gaseous giant (Jovian) planet formation; the origins of comets and Pluto-like objects, and the potential impact (pardon the pun) of these outer solar system planet-building processes on terrestrial (Earth-like) planet formation and the emergence of life on Earth itself; and the origins and compositions of planetary atmospheres.

Work on these and other fundamental aspects of star and planet formation is certain to accelerate in coming years, especially given the high priority placed on exoplanet studies in contemporary astronomy (as reflected in the Astro2010 Decadal Survey). Further ground-breaking discoveries — such as the first-time detection via direct imaging of a multiple exoplanet system (specifically, the four “warm,” gas giants orbiting the nearby young star HR 8799; Marois et al., Science, Nov. 2008) — are most likely to emerge from studies of those young stars that are nearest Earth. Hence, Kastner and his team seek to advance the understanding of the origins of Jovian exoplanets via an intensive observing campaign targeting recently identified examples of actively accreting young stars (so-called “classical T Tauri stars”) whose dusty, gaseous disks are bombarded by intense high-energy radiation fields from the central stars. Kastner and team are focusing primarily on those rare examples of classical T Tauri stars found within ∼ 100 pc.


Figure 1: First results from the GALNYSS project include the identification of roughly 50 new candidate members of the Tucana-Horologium Association of nearby, young stars. The Tuc-Hor group lies at a mean distance of ~150 light years, and its member stars are of age ~10–40 million years. The positions and proper motions of the newly GALNYSS-identified Tuc-Hor member candidates (red circles) are here shown superimposed on the same data for previously known member stars (blue squares). From Rodriguez et al. (2013).

The GALNYSS project:

Under NASA funding, Kastner and collaborators at UCLA and Universidad de Chile have developed a new method to identify nearby, low-mass stars with ages 10–100 Myr using archival GALEX (UV) and 2MASS (near-IR) data. We have now used this new UV/optical/IR-based method to compile a comprehensive list of ~2000 candidate young, late-type stars within ~100 pc of Earth (Rodriguez et al. 2013). We have named this comprehensive search the Galex Near-Young Star Survey (GALNYSS). With the list of nearby, young stars resulting from GALNYSS, we can begin to address a number of questions raised over the past decade concerning nearby young stars. Of primary interest are the luminosity and (hence) initial mass functions of the recently discovered young (age 10–100 Myr) groups within ~100 pc. In particular, by exploiting the sensitivity of Galex, we have discovered a large number of low-mass (M dwarf) stars that were too faint to be picked up by the ROSAT (X-ray) All-Sky Survey. These new detections of nearby, young M stars include ~50 new candidate members of the Tucana-Horologium Association (see Fig. 1).

Multiwavelength young star cluster studies at Protostars & Planets VI:

In the summer of 2013, AST program Ph.D. student David Principe attended the Protostars and Planets conference in Heidelberg, a small city nestled in the hills of Germany. This international conference was attended by over 800 astronomers and is one of the largest astronomical meetings for star and planet formation in the world. Principe presented his research on magnetic activity during the earliest stages of star formation. The presence and strength of magnetic fields likely influence the collapse of molecular clouds and thus, how stars and planets first form. Principe described the results of a multiwavelength campaign (e.g., radio, infrared, and X-ray observations) of two star-forming regions in Orion that he has conducted with G. Sacco (formerly a LAMA postdoc, now at Arcetri Observatory) and colleagues from the Harvard-Smithsonian Center for Astrophysics. These findings suggest that magnetic activity, as probed via high-energy X-ray emission, can be variable on timescales from days to years even in the earliest stages of young stars. Such X-ray variability may influence the earliest formation of circumstellar disks and/or planets.

Figure 2: A compendium of spectra of all molecules detected in the IRAM 30-meter telescope spectral line survey of the planet-forming disk orbiting the young star LkCa 15. From Punzi et al. (2014, in prep.).

Radio molecular line survey of a protoplanetary disk:

A team led by Kastner and IPAG (France) colleagues P. Hily-Blant and T. Forveille has conducted the first comprehensive mm-wave molecular emission line survey of the circumstellar disk orbiting the nearby, pre-main sequence (T Tauri) star LkCa 15. The outer disk orbiting LkCa 15 is chemically rich, with numerous previous detections of molecular emission lines revealing a significant gas mass. Hence, LkCa 15 is an excellent target for an unbiased radio spectroscopic survey intended to produce a full census of the detectable molecular species within an evolved, protoplanetary disk. Our survey of LkCa 15 was conducted with the Institute de Radioastronomie Millimtrique (IRAM) 30 meter telescope over the 1.1-1.4 mm wavelength range. AST program Ph.D. student Kristina Punzi is leading the analysis of these line survey data. Her work demonstrates the value of comprehensive single-dish line surveys in guiding future high resolution interferometric imaging by ALMA of protoplanetary disks orbiting T Tauri stars. Punzi’s preliminary results (see Fig. 2) were presented at the 221st American Astronomical Society meeting in January of 2013 and the Astronomical Society of New York meeting in October of 2013.

A multiwavelength spectroscopic study of the disk orbiting V4046 Sgr:

In collaboration with G. Sacco (Arcetri Observatory) and LAMA postdoc Ben Sargent, AST program Ph.D. student Valerie Rapson is conducting an infrared study of the young (age ~20 Myr), nearby (~250 light years distant) binary system V4046 Sagittarii to understand the contents and structure of the planet-forming disk around the twin young suns. Using Spitzer and Herschel spectroscopy, Rapson determined the gas and dust content of the planet forming region of the V4046 Sgr disk and discovered a plethora of lines of water, carbon monoxide, and other volatile molecules. Using newly obtained spectroscopic data from the Infrared Telescope Facility in Mauna Kea, HI, Rapson and LAMA-sponsored summer REU student C.T. Smith were able to re-analyze the spectral types (hence temperatures) of the stars in the system, and thereby demonstrate that infrared spectroscopy alone is not sufficient to determine a young star’s temperature. These projects were presented at the From Stars to Life conference in Gainesville, FL and the Astronomical Society of NY meeting in Schenectady, NY, and dovetail with Kastner’s ongoing investigations of the V4046 Sgr system (e.g., Rosenfeld et al. 2013).

Molecules in disks around T Tauri stars:

LAMA postdoc Sargent led a team analyzing spectra of a small sample of T Tauri stars, i.e., young stars with orbiting dust and gas disks that may evolve into planetary systems. In Spitzer Space Telescope Infrared Spectrograph (IRS) 5–7.5 micron wavelength spectra, these T Tauri stars shows emission from water vapor and absorption from other gases in these stars protoplanetary disks. Some show an emission feature at 6.6 microns due to warm (>500 K) water vapor, while others show an absorption band, peaking in strength at 5.6–5.7 microns. For some, this absorption band is consistent with gaseous formaldehyde (H2CO), and, for others, it is consistent with gaseous formic acid (HCOOH). The water vapor integrated flux at 6.6 microns in low resolution Spitzer-IRS spectra correlates well with water line fluxes at 17 microns in high resolution Spitzer-IRS spectra measured by Najita et al (2013). Spitzer-IRS observed many more protoplanetary disks in low resolution than high resolution, so the 6.6 micron emission is a good tracer of water vapor emission in protoplanetary disks. Modeling of these stars spectra suggests these gases are present in the inner few AU of their host disks, consistent with recent studies of infrared spectra showing gas in protoplanetary disks.

Mass-losing evolved stars and planetary nebulae 

When the Sun’s core reservoir of nuclear fuel is exhausted roughly 5 billion years in the future, it will expand to become a red giant almost 10,000 times more luminous than at present. This “monster” Sun will engulf the Earth and perhaps even Mars and Jupiter. It will eventually shed its outer atmosphere, exposing this ejected gas to the still-white-hot solar core and thereby perhaps generating a “planetary nebula”. Such planetary nebulae (PNe) and their immediate stellar antecedents -- so-called asymptotic giant branch (AGB) stars -- have served as astrophysicists’ laboratories for more than a century, providing readily accessible (to observations) examples of astrophysical plasma and shock processes, essential tests of theories of stellar evolution, and unique insight into the origin of the heavy elements in the universe and on Earth.

Long thought to signify the transition of single stars from red giant to white dwarf evolutionary stages, PNe present a dizzying variety of appearances in optical and near-infrared imaging: round; elliptical; bipolar; highly point-symmetric; chaotic and clumpy (see Figure, below). The physical mechanisms responsible for this PN morphological menagerie — and, in particular, for the evident transformation from a quasi-spherical stellar winds during the progenitor star AGB phase to nonspherical or even highly collimated outflow during the PN phase — have been the subject of intense interest and hot debate among PN researchers over the past two decades. Kastner, Montez, and their teams have been pursuing multiwavelength studies designed to distinguish between the various theories proposed to explain the AGB to PN reshaping process. Some of these ideas involve a binary companion to the mass-losing red giant star; some involve strong magnetic fields; some involve a combination of the two.

Mass loss from evolved stars in the Galactic Bulge:

In 2013, LAMA postdoc Sargent continued his NASA-funded studies of rapidly mass-losing red giants (i.e., asymptotic giant branch, or AGB, stars) in the central Bulge region of our Galaxy. Sargent et al. (2013) detected three of eight AGB stars in the Galactic bulge that they observed in the CO J = 2-1 line using the Submillimeter Array. Combining these CO data with observations at infrared wavelengths constraining dust mass loss from these stars, they determined the gas-to-dust ratios of the Galactic Bulge stars for which CO emission is detected. The derived gas/dust ratios lie in the range 200-300, i.e., somewhat larger than characteristic of the interstellar medium in the vicinity of the Sun.

The Chandra Planetary Nebula Survey:

Planetary nebulae represent a brief, spectacular phase late in the final phases of the life cycle of a Sun-like star, between its red giant and white dwarf stages. In 2013, LAMA's Kastner and inaugural RIT AST Ph.D. recipient Rudy Montez (presently a Bridge Postdoctoral Fellow at Vanderbilt University) continued to lead an international team of two dozen astronomers that is conducting a systematic Chandra X-ray imaging survey of nearby, well-studied planetary nebulae (“ChanPlaNS”; see the Spring 2013 Chandra Newsletter). ChanPlaNS began as a Large Program in Chandra’s Cycle 12, with observations of 21 objects. These imaging data were combined with archival data to yield an initial sample of 35 objects, i.e., roughly a quarter of the known planetary nebulae within roughly 4000 light years of the Sun. ChanPlaNS continued in 2013 with an allocation of more than a week of Chandra observing time in Cycle 14 to observe another 24 relatively compact objects, chosen so as to complete a survey of the youngest planetary nebulae in the solar neighborhood. AST/LAMA Ph.D. student Marcus Freeman, who is developing a thesis based on X-rays from planetaries, spearheaded RIT’s ChanPlaNS efforts in 2013. The combined ChanPlaNS and Herschel Planetary Nebula survey (HerPlaNS) teams met near Madrid, Spain in February 2013 to forge plans for future analyses and observation and theoretical programs aimed at building on the highly successful ChanPlaNS and HerPlaNS projects.

Asymmetrical Planetary Nebulae VI:

Kastner served on the Scientific Organizing Committee for the sixth meeting on Asymmetrical Planetary Nebulae (APN VI), held in November 2013 in Playa del Carmen, Mexico. Freeman and Montez presented the most recent results from ChanPlaNS at APN VI. Other topics at the meeting included 3-D simulations of the growth of planetary nebulae and the role of magnetic fields and binary companions in generating various nebula structures.