M. F. A'Hearn

Loyola University Maryland, Baltimore, Maryland, United States

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Publications (622)1084.3 Total impact

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    ABSTRACT: The close encounter of Comet C/2013 A1 (Siding Spring) with Mars on October 19, 2014 presented an extremely rare opportunity to obtain the first flyby quality data of the nucleus and inner coma of a dynamically new comet. However, the comet's dust tail potentially posed an impact hazard to those spacecraft. To characterize the comet at large heliocentric distances, study its long-term evolution, and provide critical inputs to hazard modeling, we imaged C/Siding Spring with the Hubble Space Telescope when the comet was at 4.58, 3.77, and 3.28 AU from the Sun. The dust production rate, parameterized by the quantity Af$\rho$, was 2500, 2100, and 1700 cm (5000-km radius aperture) for the three epochs, respectively. The color of the dust coma is 5.0$\pm$0.3$\%$/100 nm for the first two epochs, and 9.0$\pm$0.3$\%$/100 nm for the last epoch, and reddens with increasing cometocentric distance out to ~3000 km from the nucleus. The spatial distribution and the temporal evolution of the dust color are most consistent with the existence of icy grains in the coma. Two jet-like dust features appear in the north-northwest and southeast directions projected in the sky plane. Within each epoch of 1-2 hour duration, no temporal variations were observed for either feature, but the PA of the southeastern feature varied between the three epochs by ~30$^\circ$. The dust feature morphology suggests two possible orientations for the rotational pole of the nucleus, (RA, Dec) = (295$^\circ\pm$5$^\circ$, +43$^\circ\pm$2$^\circ$) and (190$^\circ\pm$10$^\circ$, 50$^\circ\pm$5$^\circ$), or their diametrically opposite orientations.
    10/2014;
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    ABSTRACT: We used the UltraViolet-Optical Telescope on board Swift to observe the dynamically young comet C/2009 P1 (Garradd) from a heliocentric distance of 3.5 AU pre-perihelion until 4.0 AU outbound. At 3.5 AU pre-perihelion, comet Garradd had one of the highest dust-to-gas ratios ever observed, matched only by comet Hale-Bopp. The evolving morphology of the dust in its coma suggests an outburst that ended around 2.2 AU pre-perihelion. Comparing slit-based measurements and observations acquired with larger fields of view indicated that between 3 AU and 2 AU pre-perihelion a significant extended source started producing water in the coma. We demonstrate that this source, which could be due to icy grains, disappeared quickly around perihelion. Water production by the nucleus may be attributed to a constantly active source of at least 75 km$^2$, estimated to be more than 20 percent of the surface. Based on our measurements, the comet lost $4x10^{11}$ kg of ice and dust during this apparition, corresponding to at most a few meters of its surface.Even though this was likely not Garradd's first passage through the inner solar system, the activity of the comet was complex and changed significantly during the time it was observed.
    The Astrophysical Journal 03/2014; 786(1). · 6.73 Impact Factor
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    ABSTRACT: We performed ultraviolet spectroscopy of Comet ISON with the Hubble Space Telescope.
    02/2014;
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  • M. F. A'Hearn, D. D. Wellnitz, R. Meier
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    ABSTRACT: Comets provide a very different but well understood environment in which to search for DIBs. Observations of occultations by cometary comae have not detected DIBs, but none were very near the nucleus, where the column density of dust is highest. We report here unidentified emission bands, centered at λ4430, very near the nucleus of comet Hyakutake. These may be vaporized forms of grain carriers or fragments of large-molecule carriers. At least two different species appear to be present based on two different spatial distributions.
    01/2014;
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    ABSTRACT: C/2012 S1 (ISON) is a dynamically new comet with a sungrazing perihelion only 1.7 solar radii from the Sun's surface, which makes it unique among the known comets. The discovery of C/ISON in September 2012 at a heliocentric distance of ~6 AU is more than one year before its perihelion in November 2013, allowing a detailed characterization as the comet moves from the frigid conditions of the outer solar system to extreme heating during its close passage near the Sun. As part of an international observing campaign, the Hubble Space Telescope (HST) imaged the comet in multiple epochs from April to October 2013 pre-perihelion to characterize its dust coma at 4.15, 3.78, and 1.50 AU. We report on our analysis of these HST images, including the dust production rate, the dust coma colors and morphology, and the rotation pole of the nucleus. The first two epochs of observations showed that C/ISON displays a color variations in its dust coma within ~5000 km from the nucleus that is best explained by the existence and sublimation of water ice grains. The pole orientation, as measured from the sunward jet, suggests a high obliquity, and indicates that the nucleus of C/ISON always faces the Sun with one hemisphere until about a week before the perihelion. We will use the observations in October to search for any evolutionary changes in the coma, and to refine the determination of the rotation pole.
    01/2014;
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    ABSTRACT: We analyzed Deep Impact High Resolution Instrument (HRI) images acquired within the first seconds after collision of the Deep Impact impactor with the nucleus of comet 9 P/Tempel 1. These images reveal an optically thick ejecta plume that casts a shadow on the surface of the nucleus. Using the 3D radiative transfer code HYPERION we simulated light scattering by the ejecta plume, taking into account multiple scattering of light from the ejecta, the surrounding nuclear surface and the actual observational geometry (including an updated plume orientation geometry that accounts for the latest 9 P/Tempel 1 shape model). Our primary dust model parameters were the number density of particles, their size distribution and composition. We defined the composition through the density of an individual particle and the ratio of its material constituents, which we considered to be refractories, ice and voids. The results of our modeling indicate a dust/ice mass ratio for the ejecta particles of at least 1. To further constrain the parameters of the model, we checked for consistency between the ejecta mass resulting from our modeling with the ejecta mass estimated by the crater formation modeling. Constraining the particle size distribution by results of other studies of the Deep Impact ejecta, we find the number density of ejecta particles equal to ~104 particles/cm3 at the base of the plume.
    Planetary and Space Science 01/2014; · 2.11 Impact Factor
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    ABSTRACT: Our goal was to characterize the distant gaseous and dust activity of comet C\2012 S1 (ISON), inbound, from observations of H2O, CO and the dust coma in the far-infrared and submillimeter domains. In this paper, we report observations undertaken with the Herschel Space Observatory on 8 & 13 March 2013 (rh = 4.54 - 4.47AU) and with the 30m telescope of Institut de Radioastronomie Millim\'etrique (IRAM) in March and April 2013 (rh = 4.45 - 4.18 AU). The HIFI instrument aboard Herschel was used to observe the H$_{2}$O $1_{10}-1_{01}$ line at 557 GHz, whereas images of the dust coma at 70 and 160 {\mu}m were acquired with the PACS instrument. Spectra acquired at the IRAM 30m telescope cover the CO J(2-1) line at 230.5 GHz. The spectral observations were analysed with excitation and radiative transfer models. A model of dust thermal emission taking into account a range of dust sizes is used to analyse the PACS maps. While H$_{2}$O was not detected in our 8 March 2013 observation, we derive a sensitive 3 $\sigma$ upper limit of QH$_{2}$O < 3.5 x 10$^{26}$ molecules/s for this date. A marginal 3.2 $\sigma$ detection of CO is found, corresponding to a CO production rate of QCO = 3.5 x 10$^{27}$ molecules/s. The Herschel PACS measurements show a clear detection of the coma and tail in both the 70 {\mu}m and 160 {\mu}m maps. Under the assumption of a 2 km radius nucleus, we infer dust production rates in the range 10 - 13 kg/s or 40 - 70 kg/s depending on whether a low or high gaseous activity from the nucleus surface is assumed. We constrain the size distribution of the emitted dust by comparing PACS 70 and 160 {\mu}m data, and considering optical data. Size indices between -4 and -3.6 are suggested. The morphology of the tail observed on 70 {\mu}m images can be explained by the presence of grains with ages older than 60 days.
    Astronomy and Astrophysics 11/2013; · 5.08 Impact Factor
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    ABSTRACT: We report results from broadband visible images of comet C/2012 S1 (ISON) obtained with the Hubble Space Telescope Wide Field Camera 3 on 2013 April 10. C/ISON's coma brightness follows a 1/{\rho} (where {\rho} is the projected distance from the nucleus) profile out to 5000 km, consistent with a constant speed dust outflow model. The turnaround distance in the sunward direction suggests that the dust coma is composed of sub-micron-sized particles emitted at speeds of tens of meters s$^{-1}$. A({\theta})f{\rho}, which is commonly used to characterize the dust production rate, was 1340 and 1240 cm in the F606W and F438W filters, respectively, in apertures <1.6" in radius. The dust colors are slightly redder than solar, with a slope of 5.0$\pm$0.2% per 100 nm, increasing to >10% per 100 nm 10,000 km down the tail. The colors are similar to those of comet C/1995 O1 (Hale-Bopp) and other long-period comets, but somewhat bluer than typical values for short-period comets. The spatial color variations are also reminiscent of C/Hale-Bopp. A sunward jet is visible in enhanced images, curving to the north and then tailward in the outer coma. The 1.6"-long jet is centered at a position angle of 291$^\circ$, with an opening angle of about 45$^\circ$. The jet morphology remains unchanged over 19 hours of our observations, suggesting that it is near the rotational pole of the nucleus, and implying that the pole points to within 30 deg of (RA, Dec) = (330$^\circ$, 0$^\circ$). This pole orientation indicates a high obliquity of 50$^\circ$-80$^\circ$.
    The Astrophysical Journal 11/2013; 779(1). · 6.73 Impact Factor
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    ABSTRACT: In February 2013, the Deep Impact Flyby (DIF) spacecraft observed comet C/2012 S1 ISON when the comet was ~4.7 AU from the Sun. As expected, the High Resolution Instrument Infrared Spectrometer (HRI-IR) did not detect the comet between 1.05 and 4.85 microns, a wavelength range where ro-vibrational bands of H2O, CO2, and CO, can be measured simultaneously. These measurements provide upper limits for the volatile activity. Additional pre-perihelion observations are scheduled for July/August 2013 when ISON is visible to the DIF, but unobservable from Earth. During this window, ISON is close to the water snow line 2.5 AU) and measurable activity is predicted. ISON, a dynamically new Oort Cloud comet, will be compared to the dynamically young comet C/2009 P1 Garradd, which was observed in March 2012. In those observations, HRI-IR detected H2O, CO2, and an unusually high abundance of CO post-perihelion at 2 AU. Results will also be compared to DI narrow-band measurements acquired in the same time period for both comets.
    10/2013;
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    ABSTRACT: We have assembled a variety of data on the early behavior of comet ISON through early June 2013, including V magnitudes from the ground and from spacecraft (SWIFT, DIFlyby), upper limits on gas from the DIFlyby, Herschel, and Hubble, and ground-based detections of CO and CN. We argue that the comet’s activity was steadily increasing from the pre-discovery observations in 2011 through late 2012. The activity then flattened and remained constant until January 2013, at which point it started to decrease and continued decreasing until earliest June. We interpret this in the classical picture (e.g., Whipple 1978 Moon and Planets 18, 343) of a dynamically new comet from the Oort cloud having a totally irradiated crust of order 3-10 meters thick from 4.5 billion years of galactic cosmic rays outside the heliosphere. The irradiated layer is released at very large distances due to the presence of free radicals and other chemically active species. As this crust is depleted the activity decreases and in early June we are awaiting the onset of “normal” cometary activity, which should be detected by fall at the latest.
    10/2013;
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    ABSTRACT: Comet C/2012 S1 (ISON) is a dynamically new comet on a sungrazing orbit. As such, C/ISON represents a unique opportunity to study both the cosmic-ray-irradiated surface, produced during the comet's long residence in the Oort cloud, and much deeper layers in the nucleus, exposed when the comet passes 1.7 solar radii from the Sun's surface at perihelion. During the first phase of our investigation, we collected broadband images of C/ISON on April 10, 2012 at a heliocentric distance of 4.15 AU, using the Hubble Space Telescope WFC3/UVIS. We used the F606W and F438W filters in three HST orbits covering a total span of ~19 hrs. The comet shows a well delineated coma in the sunward direction extending about 2" from the nucleus, and a dust tail at least 25" long. The coma has an average red color of 5%/0.1 micron within 1.6" from the nucleus, becoming redder towards the tail. Both the color and the size of the coma in the sunward direction are consistent with outflow of micron sized dust. Broadband photometry yielded Afρ of 1376 cm at 589 nm, and 1281 cm at 433 nm, measured with a 1.6" radius aperture. The total brightness of the comet within a 0.12" radius aperture remained unchanged within 0.03 mag for the entire duration of the observations. A well defined sunward jet is visible after removing the 1/ρ brightness distribution. The jet is centered at position angle 290 deg (E of Celestial N), with a cone angle of 45 deg, a projected length of 1.6", and a slight curvature towards the north near the end. No temporal change in the morphology is observed, suggesting the jet is circumpolar. Under this assumption, the jet’s apparent position constrains the rotational pole to lie within 30 deg of (RA, Dec) = (330, 0), and an obliquity of 50-80 deg. Preliminary analysis using a coma-nucleus separation technique suggests a nuclear radius less than 2 km. The survival of such a small nucleus during its sungrazing perihelion is certainly questionable.
    10/2013;
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    ABSTRACT: The Deep Impact flyby spacecraft (DI) is currently operating as a remote observatory, studying bright comets from a different perspective than can be achieved from Earth. Images are obtained with the Medium Resolution Imager (MRI) using a broadband CLEAR filter to capture the continuum, and narrowband filters to capture OH, CN and C2 gas bands (Farnham et al., Icarus 147, 180, 2000). Sequences consist of continuum images sampled every 15 minutes, interspersed with gas-band images that are sampled at least every hour. These sequences continue for up to 6 days, providing a continuous baseline of high-frequency observations. Comet ISON (C/2012 S1) is a dynamically new comet in a sungrazing orbit (perihelion less than 3 solar radii) that was discovered at > 6 AU from the Sun. These circumstances offer a unique opportunity to characterize the physical properties and progression of activity of this comet on its first passage into the solar system, which can be done over a wide range of heliocentric distances up to and including its close approach to the Sun. DI observed ISON in January 2013, when the comet was at a heliocentric distance of 5 AU. A second window of opportunity occurs between July and September 2013, as the comet crosses the ice line from 3 to 2 AU. This window also covers a segment of the orbit where the comet is not easily observable from Earth. Comet Garradd (C/2009 P1) is a long-period comet that was observed by DI between February 20 and April 9 2012, while the comet moved from 1.7 to 2.1 AU from the Sun. Among other results, the DI observations show rotational variability and coma morphology at levels undetected from the Earth (Farnham et al. 2013, in prep). We will present results from the analysis of the ISON and Garradd observations, including cometary variability, coma morphology, and production rates, and will discuss how the analysis fits in to the broader context for understanding the comet's development and evolution.
    10/2013;
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    ABSTRACT: The set of comets observed by SOHO and STEREO provides us with a large (>2500 comets) and unique database for studying cometary properties. Sungrazing comets are discovered in SOHO or STEREO images on average every few days, with individual comets typically observable for up to a few days. We compiled photometry of 23 comets observed simultaneously by the same telescope on both STEREO spacecraft to construct the first dust scattering phase function ever directly computed from simultaneous observations of the same object from two vantage points, thus removing uncertainty caused by changing heliocentric distance between observations. The collective dust scattering phase function spans phase angles from 28-153 deg and agrees reasonably well with the theoretical curve from Marcus 2007 (ICQ 29, 39), however, individual comets deviate from the predicted curve by varying amounts during their apparition. This may suggest that the dust properties of individual comets change on the timescale of hours due to the dramatically different heliocentric distance or that the number of dust grains in the coma is changing due to nucleus activity, rotation, and/or erosion. We have also begun a study of the dust tails of selected well-observed comets in our database. This project utilizes the 3-D aspects of the combined SOHO and STEREO dataset to constrain the dust properties and time of release better than is possible with observations from a single location. We will present ongoing results of these investigations and place them into the wider context of sungrazing comet studies. We will also present new results of our ongoing observing campaign of dynamically new sungrazing Comet ISON (C/2012 S1) at Lowell Observatory. We imaged ISON regularly from January-June 2013 with Lowell Observatory’s 4.3-m Discovery Channel Telescope and 1.1-m Hall Telescope. We observed a sunward-facing coma enhancement in R-band images in March, April, and May that is similar in orientation and extent as that reported by Li et al. 2013 (CBET 3496) with the Hubble Space Telescope. We will resume observations when ISON re-emerges from solar conjunction in September and will present first results at the meeting.
    10/2013;
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    ABSTRACT: CBET 3680 available at Central Bureau for Astronomical Telegrams.
    Central Bureau Electronic Telegrams. 10/2013;
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    ABSTRACT: Deep Impact acquired a unique rotational data set during its close flyby of comet 103/P Hartley 2 (DIXI mission, November 4, 2010). The HRI-IR spectrometer (1.05-4.85 microns) monitored the coma throughout the encounter acquiring infrared scans every 2 hr over the 18 hr period prior to closest approach and every 30 min for 2 days after closest approach. Water vapor (2.7 microns), carbon dioxide (4.3 microns), and bulk organics 3.4 microns) were the dominant emission bands detected in these spectra and their distribution was found to be highly asymmetric and variable. In particular, the distribution maps from the 8 hrs following closest approach half of the dominant 18.4 hr rotation period) are unique with spatial resolutions ranging from 0.2-3.5 km/pixel. These data allow us to explore correlations among the volatiles and the role of extended coma sources. These data will also help to quantify the heterogeneity of the outgassing and better locate specific source regions on the nucleus of Hartley 2.
    10/2013;
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    ABSTRACT: The collision of an impactor released by the Deep Impact spacecraft with comet 9P/Tempel 1 produced an ejecta cloud composed of material excavated from the comet nucleus. This surprisingly thick cloud completely obscured the impact crater and cast a shadow on the surface of the nucleus. Analysis of the images taken by the Deep Impact Medium Resolution Instrument (MRI) showed that the shadow had a complex structure, revealing variations in optical thickness within the cloud. Even over the period a few seconds, the brightness distribution within the shadow changed with time, reflecting density and/or compositional variations of the dust in the ejecta cloud. We model the scattering of sun light by the ejecta cloud to reproduce the shadow structure and its change with time. The modeling is based on the 3D radiative transfer code HYPERION (Robitalle, A&A, 536, A79, 2011). Following Richardson et al. (Icarus, 191, 176, 2007), the ejecta cloud is presented as an oblique, hollow cone. The cone is populated with dust particles whose properties, primarily composition and number density, we adjust to get the best fit to the brightness of the ejecta and the shadow. We assume that the scattering by the ejecta dust follows the Henyey-Greenstein phase function and use Mie theory to determine single scattering albedo (SSA) and extinction cross-section, averaged over size distribution. The best fit (although still tentative) could reproduce the observed peak brightness of the ejecta cone (10 W/(m2 sr µm)) and of the surface covered by the shadow (0.2 W/(m2 sr µm)). It was obtained using number density of 10-3 particles/cm3, power law size distribution with power -3, and particles composed of a mixture of equally represented amorphous carbon, silicates and organics (composition of measured comet Halley refractories) - 3.3%, ice - 29.7% and voids - 67%, that gave SSA of 0.641. Temporal and spatial variations in the properties of the dust most likely reflect variations of the structure and composition of the nucleus of Comet Tempel 1, thus providing an insight into the comet interior. The work is supported by NASA PMDAP grant #NNX10AP31G.
    10/2013;

Publication Stats

4k Citations
1,084.30 Total Impact Points

Institutions

  • 1983–2014
    • Loyola University Maryland
      Baltimore, Maryland, United States
  • 1979–2014
    • University of Maryland, College Park
      • Department of Astronomy
      Maryland, United States
  • 2008
    • University of Illinois, Urbana-Champaign
      • Department of Astronomy
      Urbana, Illinois, United States
    • The Catholic University of America
      • Department of Physics
      Washington, Washington, D.C., United States
  • 1995–2007
    • Space Telescope Science Institute
      Baltimore, Maryland, United States
    • University of Florida
      Gainesville, Florida, United States
  • 2006
    • SAIC
      Chantilly, Virginia, United States
  • 1985–2006
    • Johns Hopkins University
      • • Applied Physics Laboratory
      • • Department of Physics and Astronomy
      Baltimore, Maryland, United States
  • 2005
    • University of Hawaiʻi at Hilo
      Hilo, Hawaii, United States
    • University of Hawaiʻi at Mānoa
      • Institute of Astronomy
      Honolulu, HI, United States
    • The University of Tokyo
      • Department of Complexity Science and Engineering
      Tokyo, Tokyo-to, Japan
  • 2004
    • University of Michigan
      Ann Arbor, Michigan, United States
  • 1998
    • Brandeis University
      Waltham, Massachusetts, United States
    • Kyung Hee University
      • Department of Astronomy and Space Science
      Seoul, Seoul, South Korea
  • 1997
    • University of Maryland, Baltimore
      Baltimore, Maryland, United States
  • 1984–1997
    • The University of Arizona
      • Department of Chemistry and Biochemistry (College of Science)
      Tucson, AZ, United States
  • 1980–1982
    • University of Oklahoma
      Norman, Oklahoma, United States