Neil Dello Russo

CO Emission from Disks around AB Aurigae and HD 141569: Implications for Disk Structure and Planet Formation Timescales

We present a comparison of CO fundamental rovibrational lines (observed in the M band near 4.7 μm) from the inner circumstellar disks around the Herbig AeBe stars AB Aur and HD 141569. The CO spatial profiles and temperatures constrain the location of the gas for both stars to a distance of less than 50 AU. The CO emission from the disk of the ~4 Myr star AB Aur shows at least two temperature components, the inner disk at a rotational temperature of 1540 ± 80 K and the outer disk at 70 ± 10 K. The hot gas is located near the hot bright inner rim of the disk and the cold gas is located in the outer disk from 8-50 AU. The relative intensities of low-J lines suggest that the cold gas is optically thick. The excitation of CO in both temperature regimes is dominated by infrared fluorescence (resonant scattering). In the more evolved disk around HD 141569, the CO is excited by UV fluorescence. The relative intensity of the CO emission lines implies a rotational temperature of 190 ± 30 K. The resulting column density is ~ 1011 cm-2, indicating approximately 1019 g of CO. The observed line profiles indicate that the inner disk has been cleared of CO gas by stellar radiation out to a minimum of 17 AU. The residual mass of CO suggests that the inner disk of HD 141569 is not in an active phase of planet building but it does not rule out the possibility that giant planet building has previously occurred.

Identification of two sources of carbon monoxide in comet Hale-Bopp

The composition of ices in comets may reflect that of the molecular cloud in which the Sun formed, or it may show evidence of chemical processing in the pre-planetary accretion disk around the proto-Sun. As carbon monoxide (CO) is ubiquitous in molecular clouds,, its abundance with respect to water could help to determine the degree to which pre-cometary material was processed, although variations in CO abundance may also be influenced by the distance from the Sun at which comets formed. Observations have not hitherto provided an unambiguous measure of CO in the cometary ice (native CO). Evidence for an extended source of CO associated with comet Halley was provided by the Giotto spacecraft, but alternative interpretations exist. Here we report observations of comet Hale–Bopp which show that about half of the CO in the comet comes directly from ice stored in the nucleus. The abundance of this CO with respect to water (12 per cent) is smaller than in quiescent regions of molecular clouds, but is consistent with that measured in proto-stellar envelopes, suggesting that the ices underwent some processing before their inclusion into Hale–Bopp. The remaining CO arises in the coma, probably through thermal destruction of more complex molecules.

ORTHO-TO-PARA RATIOS OF WATER AND AMMONIA IN COMET C/2001 Q4 (NEAT): COMPARISON OF NUCLEAR SPIN TEMPERATURES OF WATER, AMMONIA, AND METHANE

Cold nuclear spin temperatures found in cometary molecules have been simply interpreted as the physical temperature when the population distribution among different nuclear spin isomers was determined through thermal equilibrium processes. However, the real meaning of cold nuclear spin temperatures is unclear due to a lack of experimental studies about the ortho-to-para ratios (OPRs) of molecules in cometary ice analogs. Here we report the nuclear spin temperatures (Tspin) of water and ammonia in comet C/2001 Q4 (NEAT). Measurements of the nuclear spin temperatures of these species and methane from previous work are all consistent with ~30 K. Consistency of the nuclear spin temperatures among different molecular species may suggest that OPRs (or abundance ratios of different nuclear spin isomers) of these molecules were last determined in thermal equilibrium. The obtained nuclear spin temperature of cometary ices is not consistent with molecular formation by hydrogen-atom addition reactions on cold grains, where the H atoms accreted from the gas phase onto grains. The condensation process on the grains might control the ortho-to-para ratios of the precometary ices, or conversion of OPRs within the ices might occur. The small diversity of the nuclear spin temperatures and lack of clear correlation between Tspin and chemical composition in several comets are consistent with the hypothesis that Tspin reflects the temperatures in the presolar nebula.

High-resolution infrared spectroscopic measurements of Comet 2P/Encke: Unusual organic composition and low rotational temperatures

Comets are classified from their orbital characteristics into two separate classes: nearly-isotropic, mainly long-period comets and ecliptic, short-period comets. Members from the former class are coming from the Oort cloud. Those of the latter class were first believed to have migrated from the Kuiper belt where they could have been accreted in situ, but recent orbital evolution simulations showed that they rather come from the trans-Neptunian scattered disc. These two reservoirs are not where the comets formed: they were expelled from the inner Solar System following interaction with the giant planets. If comets formed at different places in the Solar System, one would expect they show different chemical and physical properties. In the present paper, I review which differences are effectively observed: chemical and isotopic compositions, spin temperatures, dust particle properties, nucleus properties ... and investigate whether these differences are correlated with the different dynamical classes. The difficulty of such a study is that long-period, nearly-isotropic comets from the Oort cloud are better known, from Earth-based observations, than the weak nearly-isotropic, short-period comets. On the other hand, only the latter are easily accessed by space missions. There are not two comets alike. These objects show an extraordinary diversity (Fig. 1.1). Besides the multiplicity of their appearance, the diversity of comets is twofold: • diversity of orbits, from which different dynamical classes of comets have 1

Evolution of H2O, CO, and CO2 production in Comet C/2009 P1 Garradd during the 2011-2012 apparition

Abstract We present analysis of high spectral resolution NIR spectra of CO and H2O in Comet C/2009 P1 (Garradd) taken during its 2011–2012 apparition with the CSHELL instrument on NASA’s Infrared Telescope Facility (IRTF). We also present analysis of observations of atomic oxygen in Comet Garradd obtained with the ARCES echelle spectrometer mounted on the ARC 3.5-m telescope at Apache Point Observatory and the Tull Coude spectrograph on the Harlan J. Smith 2.7-m telescope at McDonald Observatory. The observations of atomic oxygen serve as a proxy for H2O and CO2. We confirm the high CO abundance in Comet Garradd and the asymmetry in the CO/H2O ratio with respect to perihelion reported by previous studies. From the oxygen observations, we infer that the CO2/H2O ratio decreased as the comet moved towards the Sun, which is expected based on current sublimation models. We also infer that the CO2/H2O ratio was higher pre-perihelion than post-perihelion. We observe evidence for the icy grain source of H2O reported by several studies pre-perihelion, and argue that this source is significantly less abundant post-perihelion. Since H2O, CO2, and CO are the primary ices in comets, they drive the activity. We use our measurements of these important volatiles in an attempt to explain the evolution of Garradd’s activity over the apparition.

EXTREMELY ORGANIC-RICH COMA OF COMET C/2010 G2 (HILL) DURING ITS OUTBURST IN 2012

We performed high-dispersion near-infrared spectroscopic observations of comet C/2010 G2 (Hill) at 2.5 AU from the Sun using NIRSPEC (R ≈ 25,000) at the Keck II Telescope on UT 2012 January 9 and 10, about a week after an outburst had occurred. Over the two nights of our observations, prominent emission lines of CH{sub 4} and C{sub 2}H{sub 6}, along with weaker emission lines of H{sub 2}O, HCN, CH{sub 3}OH, and CO were detected. The gas production rate of CO was comparable to that of H{sub 2}O during the outburst. The mixing ratios of CO, HCN, CH{sub 4}, C{sub 2}H{sub 6}, and CH{sub 3}OH with respect to H{sub 2}O were higher than those for normal comets by a factor of five or more. The enrichment of CO and CH{sub 4} in comet Hill suggests that the sublimation of these hypervolatiles sustained the outburst of the comet. Some fraction of water in the inner coma might exist as icy grains that were likely ejected from nucleus by the sublimation of hypervolatiles. Mixing ratios of volatiles in comet Hill are indicative of the interstellar heritage without significant alteration in the solar nebula.

Far-ultraviolet Spectroscopy of Recent Comets with the Cosmic Origins Spectrograph on the Hubble Space Telescope

Since its launch in 1990, the Hubble Space Telescope (HST) has served as a platform with unique capabilities for remote observations of comets in the far-ultraviolet region of the spectrum. Successive generations of imagers and spectrographs have seen large advances in sensitivity and spectral resolution enabling observations of the diverse properties of a representative number of comets during the past 25 years. To date, four comets have been observed in the far-ultraviolet by the Cosmic Origins Spectrograph (COS), the last spectrograph to be installed in HST, in 2009: 103P/Hartley 2, C/2009 P1 (Garradd), C/2012 S1 (ISON), and C/2014 Q2 (Lovejoy). COS has unprecedented sensitivity, but limited spatial information in its 2.5 arcsec diameter circular aperture, and our objective was to determine the CO production rates from measurements of the CO Fourth Positive system in the spectral range of 1400 to 1700 A. In the two brightest comets, nineteen bands of this system were clearly identified. The water production rates were derived from nearly concurrent observations of the OH (0,0) band at 3085 A by the Space Telescope Imaging Spectrograph (STIS). The derived CO/H2O production rate ratio ranged from ~0.3% for Hartley 2 to ~22% for Garradd. In addition, strong partially resolved emission features due to multiplets of S I, centered at 1429 A and 1479 A, and of C I at 1561 A and 1657 A, were observed in all four comets. Weak emission from several lines of the H2 Lyman band system, excited by solar Lyman-alpha and Lyman-beta pumped fluorescence, were detected in comet Lovejoy.

Evolution of H2O production in comet C/2012 S1 (ISON) as inferred from forbidden oxygen and OH emission

Abstract We present H2O production rates for comet C/2012 S1 (ISON) derived from observations of [O I] and OH emission during its inbound leg, covering a heliocentric distance range of 1.8–0.44 AU. Our production rates are in agreement with previous measurements using a variety of instruments and techniques and with data from the various observatories greatly differing in their projected fields of view. The consistent results across all data suggest the absence of an extended source of H2O production, for example sublimation of icy grains in the coma, or a source with spatial extent confined to the dimensions of the smallest projected field of view (in this case 1.2 AU, which then decreased to about half this value from Rh = 1.2–0.9 AU. This was followed by a rapid increase in active area at about Rh = 0.6 AU, corresponding to the first of three major outbursts ISON experienced inside of 1 AU. The combination of a detected outburst in the light curve and rapid increase in active area likely indicates a major nucleus fragmentation event. The 5–10 km2 active area observed outside of Rh = 0.6 AU is consistent with a 50–100% active fraction for the nucleus, larger than typically observed for cometary nuclei. Although the absolute value of the active area is somewhat dependent on the thermal model employed, the changes in active area observed are consistent among models. The conclusion of a 50–100+% active fraction is robust for realistic thermal models of the nucleus. However the possibility of a contribution of a spatially unresolved distribution of icy grains cannot be discounted. As our [OI]-derived H2O production rates are consistent with values derived using other methods, we conclude that the contribution of O2 photodissociation to the observed [O I] emission is at most 5–10% that of the contribution of H2O for ISON. This is consistent with the expected contribution of O2 photodissociation if O2/H2O  ∼  4%, meaning [O I] emission can still be utilized as a reliable proxy for H2O production in comets as long as O2/H2O  ≲  4%, similar to the abundance measured by the ROSINA instrument on Rosetta at comet 67P/Churyumov–Gerasimenko.