Observational diagnostics of gas in protoplanetary disks

Protoplanetary disks are composed primarily of gas (99% of the mass). Nevertheless, relatively few observational constraints exist for the gas in disks. In this review, I discuss several observational diagnostics in the UV, optical, near-IR, mid-IR, and (sub)-mm wavelengths that have been employed to study the gas in the disks of young stellar objects. I concentrate in diagnostics that probe the inner 20 AU of the disk, the region where planets are expected to form. I discuss the potential and limitations of each gas tracer and present prospects for future research.Comment: Review written for the proceedings of the conference "Origin and Evolution of Planets 2008", Ascona, Switzerland, June 29 - July 4, 2008. Date manuscript: October 2008. 17 Pages, 6 graphics, 134 reference

The composition of the protosolar disk and the formation conditions for comets

Conditions in the protosolar nebula have left their mark in the composition of cometary volatiles, thought to be some of the most pristine material in the solar system. Cometary compositions represent the end point of processing that began in the parent molecular cloud core and continued through the collapse of that core to form the protosun and the solar nebula, and finally during the evolution of the solar nebula itself as the cometary bodies were accreting. Disentangling the effects of the various epochs on the final composition of a comet is complicated. But comets are not the only source of information about the solar nebula. Protostellar disks around young stars similar to the protosun provide a way of investigating the evolution of disks similar to the solar nebula while they are in the process of evolving to form their own solar systems. In this way we can learn about the physical and chemical conditions under which comets formed, and about the types of dynamical processing that shaped the solar system we see today. This paper summarizes some recent contributions to our understanding of both cometary volatiles and the composition, structure and evolution of protostellar disks.Comment: To appear in Space Science Reviews. The final publication is available at Springer via http://dx.doi.org/10.1007/s11214-015-0167-

Simulating the Performance of a Fourier Transform Imaging Spectrometer on NGST

Due to its simultaneous deep imaging and integral field spectroscopic capability, an Imaging Fourier Transform Spectrograph (IFTS) is ideally suited to the NGST mission, and offers opportunities for tremendous scientific return in many fields of astrophysical inquiry. We describe the operation and quantify the advantages of an IFTS for space applications. We present the expected signal-to-noise performance of an IFTS on NGST and show that its flexible imaging and spectroscopic capabilities can execute efficiently a substantial fraction of the design reference mission. We have built a high-fidelity model that simulates the interferometric data cubes produced by a space-borne IFTS. This artificial data is invaluable because it allows us to visualize directly the data products and performance of an IFTS for comparison with other instruments. We present simulations of deep ( =~ 10(5) s) NGST IFTS observations of rich star fields and the distant Universe. These simulations demonstrate that an IFTS on NGST will be able to spectroscopically classify stars in crowded fields approaching the confusion limit, find and classify distant supernovae on the basis of their spectral signatures alone in single-epoch images, and identify young, forming super star clusters out to redshifts of z =~ 12

First Observations with the LLNL Optical Imaging Fourier Transform Spectrometer

We present the results of the first observing run with an optical imaging Fourier transform spectrometer (FTS). We have designed and fabricated this FTS for low-background astronomical use as a testbed for a proposed imaging FTS for the Next Generation Space Telescope (NGST). The relatively low background in the optical allows us to mimic the long dwell, step-scan operation of the proposed infrared NGST FTS. In this first data set, we have demonstrated the operation of the system as a multi-band camera and as a medium-resolution 3D spectrometer. Our testbed FTS reflects our current design for the NGST FTS (IFIRS). It is a four-port (two input, two output) Michelson interferometer with two 45 degree, self-compensating beamsplitters and cube-corner retro-reflectors. This system was taken to the 1.5-m McMath-Pierce Solar Observatory (MPSO) in March 1999. MPSO provides a good facility for prototyping astronomical instruments with a horizontal focal plane projected onto a (de)rotating table. We collected data from one output port with an off-the-shelf PixelVision CCD camera with a 1024x1024, thinned SITe chip thermoelectrically cooled to 235K. Our final platescale was about 0.5 arcsec/pixel with an unvignetted field of about 4x4 arcmin. We collected imaging spectroscopy with resolutions of a few to 500 of well-known objects including globular clusters, open clusters, spiral galaxies, elliptical galaxies, and nebular regions. We describe our data reduction procedures with emphasis on the unique aspects of imaging FTS data. We present color-magnitude diagrams of star clusters to demonstrate the utility of the imaging FTS as a camera and compare the signal-to-noise performance with theoretical models and filter-based camera performance. Finally, we present datacubes demonstrating the ability of the imaging FTS to yield ``a spectrum for every pixel''

IFIRS: an Imaging Fourier Transform Spectrometer for NGST

To accomplish the scientific objectives of NGST, the observatory must be equipped with instruments suitable for panchromatic observations across the 1-15 mu m spectral region on the faintest detectable objects. A wide-field imaging spectrometer that is efficient in the confusion limit, which may occur in deep field images, will maximize the scientific return and opportunities for serendipity from NGST. An imaging Fourier transform spectrometer (IFTS) supports these requirements in a low-cost, efficient instrument package that functions as an electronically programmable infrared filter with both imaging and spectroscopic capability. The conceptual design of the Integral Field Infrared Spectrograph (IFIRS) is an imaging FTS configured as a 4-port Michelson interferometer. The added ports are obtained by the use of cube-corner retroreflectors. A 4-port design delivers complementary symmetric and antisymmetric interferograms to the primary and secondary focal plane assemblies (FPAs). In this design, the object field of the complementary input is also imaged and superimposed on each image of the primary input. In operation, when observing the sky in the primary input, the secondary input would be fed with a cold blackbody having negligible radiance. The final interferogram is constructed from the difference between the two outputs (which is therefore also immune to common mode noise) while the normalized ratio of the difference to the sum of the two outputs serves to compensate for temporal variations in the object radiance, and may reveal systematic variations due to telescope throughput or detector drifts. The interferometer aperture, field angle, beam waist control, beamsplitter/beamcombiner co-planar alignment, maximum optical frequency and maximum resolution have been traded at a conceptual level of detail. These tradeoffs suggest that a 12 cm beam splitter diameter is sufficient to accept the throughput of an 8 m primary over a 5.'3 x 5.'3 square field of view

Water-rich Disks around Late M Stars Unveiled: Exploring the Remarkable Case of Sz 114

We present an analysis of the JDISCS JWST/MIRI-MRS spectrum of Sz 114, an accreting M5 star surrounded by a large dust disk with a shallow gap at ∼39 au. The spectrum is molecule-rich; we report the detection of water, CO, CO2, HCN, C2H2, and H2. The only identified atomic/ionic transition is from [Ne II] at 12.81 μm. A distinct feature of this spectrum is the forest of water lines with the 17.22 μm emission surpassing that of most mid-to-late M star disks by an order of magnitude in flux and aligning instead with disks of earlier-type stars. Moreover, the flux ratios of C2H2/H2O and HCN/H2O in Sz 114 also resemble those of earlier-type disks, with a slightly elevated CO2/H2O ratio. While accretional heating can boost all infrared lines, the unusual properties of Sz 114 could be explained by the young age of the source, its formation under unusual initial conditions (a large massive disk), and the presence of dust substructures. The latter delays the inward drift of icy pebbles and helps preserve a lower C/O ratio over an extended period. In contrast, mid-to-late M-star disks—which are typically faint, small in size, and likely lack significant substructures—may have more quickly depleted the outer icy reservoir and already evolved out of a water-rich inner disk phase. Our findings underscore the unexpected diversity within mid-infrared spectra of mid-to-late M-star disks, highlighting the need to expand the observational sample for a comprehensive understanding of their variations and thoroughly test pebble drift and planet formation models. © 2023 Institute of Physics Publishing. All rights reserved.Open access journalThis item from the UA Faculty Publications collection is made available by the University of Arizona with support from the University of Arizona Libraries. If you have questions, please contact us at [email protected]

JWST Reveals Excess Cool Water near the Snow Line in Compact Disks, Consistent with Pebble Drift

Previous analyses of mid-infrared water spectra from young protoplanetary disks observed with the Spitzer-IRS found an anticorrelation between water luminosity and the millimeter dust disk radius observed with ALMA. This trend was suggested to be evidence for a fundamental process of inner disk water enrichment proposed decades ago to explain some properties of the solar system, in which icy pebbles drift inward from the outer disk and sublimate after crossing the snow line. Previous analyses of IRS water spectra, however, were uncertain due to the low spectral resolution that blended lines together. We present new JWST-MIRI spectra of four disks, two compact and two large with multiple radial gaps, selected to test the scenario that water vapor inside the snow line is regulated by pebble drift. The higher spectral resolving power of MIRI-MRS now yields water spectra that separate individual lines, tracing upper level energies from 900 to 10,000 K. These spectra clearly reveal excess emission in the low-energy lines in compact disks compared to large disks, demonstrating an enhanced cool component with T ≈ 170-400 K and equivalent emitting radius R eq ≈ 1-10 au. We interpret the cool water emission as ice sublimation and vapor diffusion near the snow line, suggesting that there is indeed a higher inward mass flux of icy pebbles in compact disks. Observation of this process opens up multiple exciting prospects to study planet formation chemistry in inner disks with JWST. © 2023. The Author(s). Published by the American Astronomical Society.Open access journalThis item from the UA Faculty Publications collection is made available by the University of Arizona with support from the University of Arizona Libraries. If you have questions, please contact us at [email protected]

GOULD’S BELT TO STARBURST GALAXIES: THE IMF OF EXTREME STAR FORMATION

Recent results indicate the stellar initial mass function is not a strong function of star–forming environment or “initial conditions ” (e.g. Meyer et al. 2000). Some studies suggest that a universal IMF may extend to sub–stellar masses (see however Briceno et al. 2002). Yet most of this work is confined to star–forming environments within 1 kpc of the Sun. In order to probe the universality of the IMF over a wider range of parameter space (metalicity, ambient pressure, magnetic field strength) new techniques are required. We begin by summarizing our approach to deriving the sub–stellar IMF down to the opacity–limit for fragmentation using NGC 1333 as an example. Next, we describe results from simulations using the observed point–spread function of the new 6.5m MMT adaptive optics system and examine the confusion–limited sensitivity to low mass stars in rich star–forming clusters out to 0.5 Mpc. We also present preliminary results from observations with this system of the W51 star–forming complex. Finally, we outline a new technique to estimate the ratio of high to low mass stars in unresolved stellar populations, such as the massive star clusters observed in interacting galaxies (e.g. Mengel et al. 2002). While evidence for variations in the IMF remains inconclusive, new studies are required to rule them out and determine whether or not the IMF is universal over the range of parameter space relevant to star–forming galaxies over cosmic time. 1