Showing papers by "Gary J. Melnick published in 2005"

Deep Impact: Observations from a worldwide Earth-based campaign

Abstract: On 4 July 2005, many observatories around the world and in space observed the collision of Deep Impact with comet 9P/Tempel 1 or its aftermath. This was an unprecedented coordinated observational campaign. These data show that (i) there was new material after impact that was compositionally different from that seen before impact; (ii) the ratio of dust mass to gas mass in the ejecta was much larger than before impact; (iii) the new activity did not last more than a few days, and by 9 July the comet's behavior was indistinguishable from its pre-impact behavior; and (iv) there were interesting transient phenomena that may be correlated with cratering physics.

Spitzer Observations of CO2 Ice Towards Field Stars in the Taurus Molecular Cloud

Abstract: We present the first Spitzer Infrared Spectrograph observations of the 15.2 micron bending mode of CO2 ice towards field stars behind a quiescent dark cloud. CO2 ice is detected towards 2 field stars (Elias 16, Elias 3) and a single protostar (HL Tau) with anabundance of ~15-20% relative to water ice. CO2 ice is not detected towards the source with lowest extinction in our sample, Tamura 17 (A_V = 3.9m). A comparison of the Elias 16 spectrum with laboratory data demonstrates that the majority of CO2 ice is embedded in a polar H2O-rich ice component, with ~15% of CO2 residing in an apolar H2O-poor mantle. This is the first detection of apolar CO2 towards a field star. We find that the CO2 extinction threshold is A_V = 4m +/- 1m, comparable to the threshold for water ice, but significantly less than the threshold for CO ice, the likely precursor of CO2. Our results confirm CO2 ice forms in tandem with H2O ice along quiescent lines of sight. This argues for CO2 ice formation via a mechanism similar to that responsible for H2O ice formation, viz. simple catalytic reactions on grain surfaces.

Spitzer observations of CO2 ice toward field stars in the taurus molecular cloud

Abstract: We present the first Spitzer Infrared Spectrograph observations of the 152 μm bending mode of CO2 ice toward field stars behind a quiescent dark cloud CO2 ice is detected toward two field stars (Elias 16 and Elias 3) and a single protostar (HL Tau) with an abundance of ~15%-20% relative to water ice CO2 ice is not detected toward the source with lowest extinction in our sample, Tamura 17 (AV = 39 mag) A comparison of the Elias 16 spectrum with laboratory data demonstrates that the majority of CO2 ice is embedded in a polar, H2O-rich ice component, with ~15% of CO2 residing in an apolar, H2O-poor mantle This is the first detection of apolar CO2 toward a field star We find that the CO2 extinction threshold is AV = 4 ± 1 mag, comparable to the threshold for water ice, but significantly less than the threshold for CO ice, the likely precursor of CO2 Our results confirm that CO2 ice forms in tandem with H2O ice along quiescent lines of sight This argues for CO2 ice formation by means of a mechanism similar to that responsible for H2O ice formation, viz, simple catalytic reactions on grain surfaces

Detection of Water in the Shocked Gas Associated with IC 443: Constraints on Shock Models

Abstract: We have used the Submillimeter Wave Astronomy Satellite (SWAS) to observe the ground-state 110 → 101 transition of ortho-H2O at 557 GHz in three of the shocked molecular clumps associated with the supernova remnant IC 443. We also observed simultaneously the 487 GHz line (3,1 → 3,2) of O2, the 492 GHz line (3P1 → 3P0) of C I, and the 550 GHz line (J = 5 → 4) of 13CO. We detected the H2O, C I, and 13CO lines toward the shocked clumps B, C, and G. In addition, ground-based observations of the J = 1 → 0 transitions of CO and HCO+ were obtained. Assuming that the shocked gas has a temperature of 100 K and a density of 5 × 105 cm-3, we derive SWAS beam-averaged ortho-H2O column densities of 3.2 × 1013, 1.8 × 1013, and 3.9 × 1013 cm-2 in clumps B, C, and G, respectively. Combining the SWAS results with our ground-based observations, we derive a relative abundance of ortho-H2O to CO in the postshock gas of between 2 × 10-4 and 3 × 10-3. On the basis of our results for H2O, published results of numerous atomic and molecular shock tracers, and archival Infrared Space Observatory (ISO) observations, we conclude that no single shock type can explain these observations. However, a combination of fast J-type shocks (~100 km s-1) and slow C-type shocks (~12 km s-1) or, more likely, slow J-type shocks (~12-25 km s-1) can most naturally explain the postshock velocities and the emission seen in various atomic and molecular tracers. Such a superposition of shocks might be expected as the supernova remnant overtakes a clumpy interstellar medium. The fast J-type shocks provide a strong source of ultraviolet radiation, which photodissociates the H2O in the cooling (T ≤ 300 K) gas behind the slow shocks and strongly affects the slow C-type shock structure by enhancing the fractional ionization. At these high ionization fractions, C-type shocks break down at speeds ~10-12 km s-1, while faster flows will produce J-type shocks. Our model favors a preshock gas-phase abundance of oxygen not in CO that is depleted by a least a factor of 2, presumably as water ice on grain surfaces. Both freezeout of H2O and photodissociation of H2O in the postshock gas must be significant to explain the weak H2O emission seen by SWAS and ISO from the shocked and postshock gas.

Study of high-performance coronagraphic techniques

Abstract: The goal of the Study of High Performance Coronagraphic Techniques project (called CoronaTech) is: 1) to verify the Labeyrie multi-step speckle reduction method and 2) to develop new techniques to manufacture soft-edge occulter masks preferably with Gaussian absorption profile. In a coronagraph, the light from a bright host star which is centered on the optical axis in the image plane is blocked by an occulter centered on the optical axis while the light from a planet passes the occulter (the planet has a certain minimal distance from the optical axis). Unfortunately, stray light originating in the telescope and subsequent optical elements is not completely blocked causing a so-called speckle pattern in the image plane of the coronagraph limiting the sensitivity of the system. The sensitivity can be increased significantly by reducing the amount of speckle light. The Labeyrie multi-step speckle reduction method implements one (or more) phase correction steps to suppress the unwanted speckle light. In each step, the stray light is rephased and then blocked with an additional occulter which affects the planet light (or other companion) only slightly. Since the suppression is still not complete, a series of steps is required in order to achieve significant suppression. The second part of the project is the development of soft-edge occulters. Simulations have shown that soft-edge occulters show better performance in coronagraphs than hard-edge occulters. In order to utilize the performance gain of soft-edge occulters. fabrication methods have to be developed to manufacture these occulters according to the specification set forth by the sensitivity requirements of the coronagraph.