Joan R. Najita

The K2 Mission: Characterization and Early Results

The K2 mission will make use of the Kepler spacecraft and its assets to expand upon Keplers groundbreaking discoveries in the fields of exoplanets and astrophysics through new and exciting observations. K2 will use an innovative way of operating the spacecraft to observe target fields along the ecliptic for the next 2-3 years. Early science commissioning observations have shown an estimated photometric precision near 400 ppm in a single 30 minute observation, and a 6-hr photometric precision of 80 ppm (both at V = 12). The K2 mission offers long-term, simultaneous optical observation of thousands of objects at a precision far better than is achievable from ground-based telescopes. Ecliptic fields will be observed for approximately 75 days enabling a unique exoplanet survey which fills the gaps in duration and sensitivity between the Kepler and TESS missions, and offers pre-launch exoplanet target identification for JWST transit spectroscopy. Astrophysics observations with K2 will include studies of young open clusters, bright stars, galaxies, supernovae, and asteroseismology.

Magnetocentrifugally driven flows from young stars and disks. 1: A generalized model

We propose a generalized model for stellar spin-down, disk accretion, and truncation, and the origin of winds, jets, and bipolar outflows from young stellar objects. We consider the steady state dynamics of accretion of matter from a viscous and imperfectly conducting disk onto a young star with a strong magnetic field. For an aligned stellar magnetosphere, shielding currents in the surface layers of the disk prevent stellar field lines from penetrating the disk everywhere except for a range of radii about pi = R(sub x), where the Keplerian angular speed of rotation Omega(sub x) equals the angular speed of the star Omega(sub *). For the low disk accretion rates and high magnetic fields associated with typical T Tauri stars, R(sub x) exceeds the radius of the star R(sub *) by a factor of a few, and the inner disk is effectively truncated at a radius R(sub t) somewhat smaller than R(sub x). Where the closed field lines between R(sub t) and R(sub x) bow sufficiently inward, the accreting gas attaches itself to the field and is funneled dynamically down the effective potential (gravitational plus centrifugal) onto the star. Contrary to common belief, the accompanying magnetic torques associated with this accreting gas may transfer angular momentum mostly to the disk rather than to the star. Thus, the star can spin slowly as long as R(sub x) remains significantly greater than R(sub *). Exterior to R(sub x) field lines threading the disk bow outward, which makes the gas off the mid-plane rotate at super-Keplerian velocities. This combination drives a magnetocentrifugal wind with a mass-loss rate M(sub w) equal to a definite fraction f of the disk accretion rate M(sub D). For high disk accretion rates, R(sub x) is forced down to the stellar surface, the star is spun to breakup, and the wind is generated in a manner identical to that proposed by Shu, Lizano, Ruden, & Najita in a previous communication to this journal. In two companion papers (II and III), we develop a detailed but idealized theory of the magnetocentrifugal acceleration process.

Disks in Transition in the Taurus Population: Spitzer IRS Spectra of GM Aurigae and DM Tauri

We present Spitzer Infrared Spectrograph (IRS) observations of two objects of the Taurus population that show unambiguous signs of clearing in their inner disks. In one of the objects, DM Tau, the outer disk is truncated at 3 AU; this object is akin to another recently reported in Taurus, CoKu Tau/4, in that the inner disk region is free of small dust. Unlike CoKu Tau/4, however, this star is still accreting, so optically thin gas should still remain in the inner disk region. The other object, GM Aur, also accreting, has ~0.02 lunar masses of small dust in the inner disk region within ~5 AU, consistent with previous reports. However, the IRS spectrum clearly shows that the optically thick outer disk has an inner truncation at a much larger radius than previously suggested, ~24 AU. These observations provide strong evidence for the presence of gaps in protoplanetary disks.

HEATING PROTOPLANETARY DISK ATMOSPHERES

We calculate the thermal-chemical structure of the gaseous atmospheres of the inner disks of T Tauri stars, starting from the density and dust temperature distributions derived by D’Alessio and coworkers in 1999. As a result of processes such as X-ray irradiation or mechanical heating of the surface layers, the gas temperature at the very top of the disk atmosphere in the neighborhood of 1 AU is of the order of 5000 K. Deep down, it drops rapidly into the range of the dust temperature, i.e., several hundred degrees kelvin. In between these upper hot and lower cool layers, there is a transition zone with gas temperatures in the range 500–2000 K. The thickness and location of this warm region depend on the strength of the surface heating. This region also manifests the basic chemical transitions of H to H2 and C + and C to CO. It is remarkable that even though the H2 transition begins first (higher up), it does not go to completion until after CO does. Consequently, there is a reasonably thick layer of warm CO that is predominantly atomic H. This thermal-chemical structure is favorable to the excitation of the fundamental and overtone bands of CO because of the large rate coefficients for vibrational excitation in H+CO as opposed to H2+CO collisions. This conclusion is supported by the recent observations of the fundamental band transitions in most T Tauri stars. We also argue that layered atmospheres of inner T Tauri disks may play an important role in understanding the observations of H2 UV fluorescence pumped from excited vibrational levels of that molecule. Possible candidates for surface heating include the interaction of a wind with the upper layers of the disk and dissipation of hydromagnetic waves generated by mechanical disturbances close to the midplane, e.g., by the Balbus-Hawley instability. Detailed modeling of the observations has the potential to reveal the nature of the mechanical surface heating that we model phenomenologically in these calculations and to help explain the nature of the gas in protoplanetary disks.

Gas in the Terrestrial Planet Region of Disks: CO Fundamental Emission from T Tauri Stars

We report the results of a high-resolution spectroscopic survey for CO fundamental emission from T Tauri stars. CO fundamental emission is frequently detected, with the likely origin of the emission in the circumstellar disk. An initial assessment of the line profiles indicates that the emission region includes the equivalent of the terrestrial planet region of our solar system, a result that suggests the utility of CO fundamental emission as a probe of disks at planet formation distances. Since fundamental emission is detected frequently from both close binary and apparently single stars, it appears that both low column density regions, such as disk gaps, and temperature inversion regions in disk atmospheres can produce significant emission. The estimated excitation temperature of the emitting gas is unexpectedly warm for the disk radii that they appear to probe. Thus, the surface gaseous component of inner disks may be significantly warmer than the surface dust component. We also detect CO emission from a transitional T Tauri star. Because fundamental emission from CO and its isotopes is sensitive to a wide range of gas masses, including masses M⊕, CO fundamental emission may prove useful in measuring the residual gas content of dissipating disks. This may be an effective way to explore the gas dissipation timescale in inner disks and to thereby place constraints on the timescale for giant planet formation.

Organic molecules and water in the planet formation region of young circumstellar disks.

The chemical composition of protoplanetary disks is expected to hold clues to the physical and chemical processes that influence the formation of planetary systems. However, characterizing the gas composition in the planet formation region of disks has been a challenge to date. We report here that the protoplanetary disk within 3 astronomical units of AA Tauri possesses a rich molecular emission spectrum in the mid-infrared, indicating a high abundance of simple organic molecules (HCN, C2H2, and CO2), water vapor, and OH. These results suggest that water is abundant throughout the inner disk and that the disk supports an active organic chemistry.

Demographics of transition objects

The unusual properties of transition objects (young stars with an optically thin inner disc surrounded by an optically thick outer disc) suggest that significant disc evolution has occurred in these systems. We explore the nature of these systems by examining their demographics, specifically their stellar accretion rates ˙ M∗ and disc masses Mdisc compared to those of accreting T Tauri stars of comparable age. We find that the transition objects in Taurus occupy a restricted region of the ˙ M∗ versus Mdisc plane. Compared to non-transition single stars in Taurus, they have stellar accretion rates that are typically ∼10 times lower at the same disc mass and median disc masses approximately four times larger. These properties are anticipated by several proposed planet formation theories and suggest that the formation of Jovian mass planets may play a significant role in explaining the origin of at least some transition objects. Considering transition objects as a distinct demographic group among accreting T Tauri stars leads to a tighter relationship between disc masses and stellar accretion rates, with a slope between the two quantities that is close to the value of unity expected in simple theories of disc accretion.

Magnetocentrifugally Driven Flows from Young Stars and Disks. V. Asymptotic Collimation into Jets

We consider the asymptotic behavior of magnetocentrifugally driven X-winds, taking into proper account the pressure balance across the interface between the open stellar field lines of the dead zone devoid of matter and the open field lines of the wind. At large distances from the source the outflow collimates into jets along the rotation axis. Being almost cylindrically symmetric even close to the source of the outflow, the density distribution appears more jetlike than the collection of streamlines, which always contains a few members that skim over the surface of the disk.

Formation and Evolution of Planetary Systems: Upper Limits to the Gas Mass in Disks Around Sun-like Stars

We have carried out a sensitive search for gas emission lines at IR and millimeter wavelengths for a sample of 15 young Sun-like stars selected from our dust disk survey with Spitzer. We have used mid-IR lines to trace the warm (300-100 K) gas in the inner disk and millimeter transitions of ^(12)CO to probe the cold (~20 K) outer disk. We report no gas line detections from our sample. Line flux upper limits are first converted to warm and cold gas mass limits using simple approximations allowing a direct comparison with values from the literature. We also present results from more sophisticated models following Gorti & Hollenbach that confirm and extend our simple analysis. These models show that the [S I] 25.23 μm line can set constraining limits on the gas surface density at the disk inner radius and traces disk regions up to a few AU. We find that none of the 15 systems have more than 0.04M_J of gas within a few AU from the disk inner radius for disk radii from 1 to ~40 AU. These gas mass upper limits even in the eight systems younger than ~30 Myr suggest that most of the gas is dispersed early. The gas mass upper limits in the 10-40 AU region, which is mainly traced by our CO data, are <2 M_⊕. If these systems are analogs of the solar system, they either have already formed Uranus- and Neptune-like planets or will not form them beyond 100 Myr. Finally, the gas surface density upper limits at 1 AU are smaller than 0.01% of the minimum mass solar nebula for most of the sources. If terrestrial planets form frequently and their orbits are circularized by gas, then circularization occurs early.

A Spitzer Survey of Mid-infrared Molecular Emission from Protoplanetary Disks. I. Detection Rates

We present a Spitzer InfraRed Spectrometer search for 10-36 μm molecular emission from a large sample of protoplanetary disks, including lines from H_(2)O, OH, C_(2)H_2, HCN, and CO2. This paper describes the sample and data processing and derives the detection rate of mid-infrared molecular emission as a function of stellar mass. The sample covers a range of spectral type from early M to A, and is supplemented by archival spectra of disks around A and B stars. It is drawn from a variety of nearby star-forming regions, including Ophiuchus, Lupus, and Chamaeleon. Spectra showing strong emission lines are used to identify which lines are the best tracers of various physical and chemical conditions within the disks. In total, we identify 22 T Tauri stars with strong mid-infrared H2O emission. Integrated water line luminosities, where water vapor is detected, range from 5 × 10^(–4) to 9 × 10^(–3) L_☉ sun, likely making water the dominant line coolant of inner disk surfaces in classical T Tauri stars. None of the five transitional disks in the sample show detectable gaseous molecular emission with Spitzer upper limits at the 1% level in terms of line-to-continuum ratios (apart from H_2), but the sample is too small to conclude whether this is a general property of transitional disks. We find a strong dependence on detection rate with spectral type; no disks around our sample of 25 A and B stars were found to exhibit water emission, down to 1%-2% line-to-continuum ratios, in the mid-infrared, while more than half of disks around late-type stars (M-G) show sufficiently intense water emission to be detected by Spitzer, with a detection rate approaching 2/3 for disks around K stars. Some Herbig Ae/Be stars show tentative H_(2)O/OH emission features beyond 20 μm at the 1%-2% level, however, and one of them shows CO_2 in emission. We argue that the observed differences between T Tauri disks and Herbig Ae/Be disks are due to a difference in excitation and/or chemistry depending on spectral type and suggest that photochemistry may be playing an important role in the observable characteristics of mid-infrared molecular line emission from protoplanetary disks.