Comet 17P/Holmes in Outburst: The Near Infrared Spectrum

arXiv:0903.1317v1 [astro-ph.EP] 7 Mar 2009 Comet 17P/Holmes in Outburst: The Near Infrared Spectrum Bin Yang, David Jewitt and Schelte J. Bus Institute for Astronomy, University of Hawaii, Honolulu, HI 96822 yangbin@ifa.hawaii.edu, jewitt@ifa.hawaii.edu Abstract Jupiter family comet 17P/Holmes underwent a remarkable outburst on UT 2007 Oct. 24, in which the integrated brightness abruptly increased by about a factor of a million. We obtained near infrared (0.8 - 4.2µm) spectra of 17P/Holmes on UT 2007 Oct. 27, 28 and 31, using the 3.0-m NASA Infrared Telescope Facility (IRTF) atop Mauna Kea. Two broad absorption bands were found in the reflectance spectra with centers (at 2µm and 3µm, respectively) and overall shapes consistent with the presence of water ice grains in the coma. Synthetic mixing models of these bands suggest an origin in cold ice grains of micron size. Curiously, though, the expected 1.5µm band of water ice was not detected in our data, an observation for which we have no explanation. Simultaneously, excess thermal emission in the spectra at wavelengths beyond 3.2µm has a color temperature of 360 ± 40 K (corresponding to a superheat factor of ∼2.0 ± 0.2 at 2.45 AU). This is too hot for these grains to be icy. The detection of both water ice spectral features and short-wavelength thermal emission suggests that the coma of 17P/Holmes has two components (hot, refractory dust and cold ice grains) which are not in thermal contact. A similarity to grains ejected into the coma of 9P/Tempel 1 by the Deep Impact spacecraft is noted. 1. Introduction As a Jupiter-family comet, 17P/Holmes is unremarkable in terms of either its dynamical prop- erties or its chemical composition (Whipple 1984). However, on UT 2007 Oct. 24, its brightness increased by about a million times in less than a day, with its apparent magnitude rising from ∼17 to ∼1.5 (Buzzi et al. 2007). In comparison, NASA’s Deep Impact into the nucleus of comet 9P/Tempel 1 in 2005 changed the brightness of the comet by a comparatively modest two magni- tudes (Fern´andez et al. 2007). The spectacular brightening of 17P echoed an earlier outburst on November 6 1892, which led to the discovery of the comet by the British amateur astronomer Edwin Holmes in London. The two outbursts, 115 years apart, are strikingly similar in the sense that both occured 5 months after the comet had passed its perihelion and in both cases the cometary coma increased dramatically in angular size and brightness. However, whereas the comet experienced a second outburst in January 1893, two months after the initial event, no such second outburst followed the October 2007 event. – 2 – Over the years, various models and hypotheses have been advanced to explain cometary out- burst phenomena (Gronkowski 2007). Some outbursts are associated with cometary splitting, in which the primary nucleus ejects discrete fragments, perhaps in response to rotational or other instabilities. Unfortunately, there is no strong evidence concerning the rotational period of 17P (Snodgrass et al. 2006) and nothing to suggest that it is close to rotational break-up. Some comets split due to strong tidal forces from nearby massive objects, usually the Sun or Jupiter (Boehnhardt 2004). However, 17P/Holmes was far from Jupiter and the Sun at the time of the outbursts, giving no reason to suspect a tidal trigger. Whipple (1984) described a scenario whereby the impact of an unseen satellite could have triggered the 1892 outburst. However, the repetitive nature of the outbursts in this comet renders this explanation implausible. A conjectural scenario is that a large fraction of 17P/Holmes is amorphous water ice in which are trapped considerable amounts of volatile molecules. When close to the Sun, elevated internal temperatures might have precipi- tated an irreversible phase transition in water ice from amorphous to crystalline. This (exothermic) transition process would release the trapped volatiles, driving up the gas pressure and eventually triggering the observed outburst (e.g. Prialnik et al. 2004). In this paper we study the material ejected from 17P via near infrared spectroscopy (0.8 - 4.2µm) obtained shortly after the start of the outburst. 2. Observations Observations were obtained using the NASA Infrared Telescope Facility (IRTF) 3-m tele- scope atop Mauna Kea, Hawaii. We observed 17P/Holmes on UT 2007 Oct. 27, 28 and 31, using SpeX (Rayner et al. 2003), a medium-resolution 0.8-5.5µm spectrograph. SpeX is equipped with a Raytheon 1024 ×1024 InSb array having a spatial scale of 0.′′15 pixel−1. Two cross-dispersed modes, known as SXD (0.8 - 2.4µm) and LXD (1.9 - 4.2µm), were used to cover an overall wavelength range from 0.8µm to 4.2µm for all our IRTF observations. The spectral resolution afforded by SpeX is dependent, in part, on the width of the spectrograph slit. To achieve high resolution, we used a 0.′′3× 15′′ slit that provided an average spectral resolving power of λ/∆λ = 2300. The slit

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