THE LONG-TERM DECAY IN PRODUCTION RATES FOLLOWING THE EXTREME OUTBURST OF COMET 17P/HOLMES

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1 The Astrophysical Journal, 138: , 2009 October C The American Astronomical Society. All rights reserved. Printed in the U.S.A. doi: / x/138/4/1062 THE LONG-TERM DECAY IN PRODUCTION RATES FOLLOWING THE EXTREME OUTBURST OF COMET 17P/HOLMES David G. Schleicher Lowell Observatory, 1400 W. Mars Hill Road, Flagstaff, AZ 86001, USA; dgs@lowell.edu Received 2009 April 8; accepted 2009 July 5; published 2009 September 1 ABSTRACT Numerous sets of narrowband filter photometry were obtained of Comet 17P/Holmes from Lowell Observatory during the interval of 2007 November 1 to 2008 March 5. Observations began 8 days following its extreme outburst, at which time the derived water production rate, based on OH measurements, was molecule s 1 and the derived proxy of dust production, A(θ)fρ, was about cm. Relative production rates for the other gas species, CN, C 2,C 3, and NH, are consistent with typical composition (based on our update to the classifications by A Hearn et al.). An exponential decay in the logarithm of measured production rates as a function of time was observed for all species, with each species dropping by factors of about after 125 days. All gas species exhibited clear trends with aperture size, and these trends are consistent with larger apertures having a greater proportion of older material that was released when production rates were higher. Much larger aperture trends were measured for the dust, most likely because the dust grains have smaller outflow velocities and longer lifetimes than the gas species; therefore, a greater proportion of older, i.e., higher production dust is contained within a given aperture. By extrapolating to a sufficiently small aperture size, we derive near-instantaneous water and dust production rates throughout the interval of observation, and also estimate values immediately following the outburst. The finite lifetime of the gas species requires that much higher ice vaporization rates were taking place throughout the observation interval than occurred prior to the outburst, likely due to the continued release of icy grains from the nucleus. The relatively small aperture trends for the gas species also imply that the bulk of fresh, excess volatiles are confined to the nucleus and near-nucleus regime, rather than being associated with the outburst ejecta cloud. A minimum of about 0.1% of the total nucleus volume was vaporized water ice, while a dust volume corresponding to at least 1% 2% was likely to have been released from the nucleus. Key words: comets: individual (17P/Holmes) techniques: photometric 1. INTRODUCTION 1.1. Background It has long been known that comets experience outbursts on a variety of timescales and a variety of magnitudes. Obtaining an understanding of causes of such outbursts is important both as a probe of the physical structure and chemical composition of the interior of individual objects, and also to provide clues as to the primordial conditions which existed at the sites of origin of these objects and resulted in these fundamental properties. Some outbursts are clearly associated with the partial or even complete breakup of a comet s nucleus, as in the recent cases of 73P/Schwassmann Wachmann 3 (partial fragmentation), Hyakutake (1996 B2; shed debris), and LINEAR (1999 S4; complete disruption). In other cases, the observed outbursts do not appear to be associated with fragmentation events. For instance, 29P/Schwassmann Wachmann 1 hiccups on an irregular basis, brightening by 2 6 mag from baseline activity. Overall, the observational evidence strongly suggests that cometary outbursts are caused by a variety of mechanisms. As the most extreme case observed in recent times, the mag brightening of Comet 17P/Holmes is expected to reveal important information on at least one class of outburst mechanisms Comet 17P/Holmes Discovered near midnight on the night of 1892 November 6/7 at about 5th magnitude, Comet 17P/Holmes (1892f = 1992 III = 1892 V1) is commonly believed to have experienced a major outburst just prior to, and resulting in, its discovery. As detailed by Kronk (2003), the comet progressively increased in size and dimmed for about 10 weeks before it experienced another outburst bringing it back up to about 8th magnitude, after which it again expanded and slowly faded from view. The extreme nature of these outbursts only became evident when Holmes was recovered during its 1899 apparition and was never brighter than about 13th magnitude. At its 1906 apparition, the brightness peaked at near 16th magnitude, and Holmes was then lost until 1964 when it was recovered at 19th magnitude (compare Kronk 1984). Brightness estimates prior to Comet Holmes extreme outburst of 2007 place it at approximately 17th magnitude, with the entire brightness increase to magnitude 2.5 occurring in less than 24 hr on October 23 and 24 (compare Santana 2007; Yoshida 2008; Sekanina 2008). At 17th magnitude, Holmes is simply too faint (by about a factor of 10) to be a viable target for our long-term narrowband photometry program for comets (compare Schleicher et al. 2007), and we therefore have no preoutburst observations. Our first data sets were obtained on 2007 November 1 (Schleicher 2007), just 8 days after reaching peak brightness, and observations were completed on 2008 March 5, by which time the opto-center was difficult to discern even though the nominal total brightness of the comet remained near 5th magnitude, due to very low contrast as the comet s apparent size approached 1.5 deg. Here, we present our narrowband photometric results for this 18-week interval during which Comet Holmes activity declined following its outburst, including production rates and abundance ratios, along with associated derived quantities. These data 1062

2 No. 4, 2009 POST-OUTBURST PHOTOMETRY OF COMET 17P/HOLMES 1063 Table 1 Observing Circumstances and Fluorescence Efficiencies for Comet 17P/Holmes UT Date ΔT a r H Δ Phase ṙ H log L/N b (erg s 1 molecule 1 ) (day) (AU) (AU) Angle ( ) (kms 1 ) OH NH CN 2007 Nov Nov Nov Dec Dec Dec Jan Mar Mar Notes. a Time from perihelion, 2007 May 4.50; the onset of the outburst occurred at about 2007 October 23.7, or ΔT = b Fluorescence efficiencies are for r H = 1 AU, and are scaled by r 2 H in the reductions. provide both an overview and context of Holmes behavior for use with the many other more specialized and shorter-interval observations from other researchers. We also discuss the size and frequency of mega-outbursts in Holmes and speculate as to their cause. 2. OBSERVATIONS AND REDUCTIONS Observations began on 2007 November 1, just 8 days following the outburst, and continued on a total of nine nights, concluding on 2008 March 5. All observations were obtained with the 42 inch (1.1 m) John S. Hall telescope at Lowell Observatory s Anderson Mesa site using a conventional, photoelectric photometer equipped with an EMI 6256 photomultiplier tube and pulse-counting electronics. Eight narrowband filters from the HB comet filter set (Farnham et al. 2000) isolated molecular emission bands of OH, NH, CN, C 2, and C 3, and reflected continuum from the dust at 3448, 4450, and 5260 Å. Atmospheric extinction coefficients and absolute instrumental calibrations were determined nightly for each filter based on measurements of comet standard stars (Farnham et al. 2000). Observational circumstances for Holmes, including the dates and time from perihelion (ΔT), and the heliocentric (r H ) and geocentric (Δ)distances, are listed in Table 1. Note that the onset of the outburst is estimated to have occurred at 2007 October 23.7, corresponding to ΔT = day. Depending on the brightness of the comet, several 10 or 15 s integrations were obtained for each filter, resulting in total integrations of s per filter; each observational set of eight filters was interspersed with or followed by separate sky measurements. A total of nine different photometer entrance apertures were used, ranging in diameter from 24 to 204 arcsec; the number of apertures and the sizes used each night depended on the time available and the need to avoid background stars. Overall, 34 sets were obtained and the UT mid-time and aperture size for each set, along with the log of the projected aperture radius (ρ) at the distance of the comet, are listed in Table 2. Data reduction followed our standard methodology as detailed in A Hearn et al. (1995), but using the coefficients and procedures specific to the HB filters (Farnham et al. 2000). Absolute continuum fluxes and continuum-subtracted emission band fluxes are given in Table 2. Also listed are the molecular column abundances, M(ρ), that are computed after applying the appropriate fluorescence efficiencies (L/N) for each gas species. Nightly L/N values are given in Table 1; due to the Swings effect these vary with heliocentric velocity (ṙ H ) for OH, NH, and CN, and, in the case of CN, also vary with heliocentric distance. Total abundances were next computed from the column abundances by applying a standard Haser model (Haser 1957) using parent and daughter Haser scalelengths tabulated in A Hearn et al. (1995) and scaled by rh 2. Gas production rates, Q, were then calculated by dividing the total abundances by the assumed daughter lifetimes also tabulated in A Hearn et al. and scaled by rh 2. For OH, we also derive the parent, i.e., water, production rate using the empirical relation Q(H 2 O; vectorial) = r 0.5 H Q(OH; Haser) from Cochran & Schleicher (1993; see also Schleicher et al. 1998). Resulting production rates are given in Table 3. As usual, we use the quantity A(θ)fρ as a proxy for dust production, as it makes no assumptions regarding the particle size distribution while the resulting value is independent of aperture size and wavelength if the dust grains have a canonical 1/ρ radial distribution and are gray in color. A(θ)fρ is the product of the dust Bond albedo at a given phase angle, A(θ), the filling factor of the dust within the aperture, f, and the projected aperture radius, ρ, (compare A Hearn et al. 1984; A Hearn et al. 1995). The resulting values are also given in Table 3. Due to the brightness of Comet Holmes in the first 10 weeks following its outburst, uncertainties associated with photon statistics are generally quite small. In particular, the continuum points and the high contrast emission bands of OH and CN have 1σ uncertainties of <3% while the somewhat lower contrast features of C 2 and C 3 have uncertainties <9%, with highest uncertainties occurring with the smallest aperture sizes (due to decreased contrast of the emission bands to the underlying continuum). NH, having the weakest emission band, has uncertainties ranging from 2% to 16% during this interval. While the total brightness of Holmes was still high at the conclusion of our observations in March, it had become a very large, low surface brightness object, resulting in much greater uncertainties for several species. Consequently, the uncertainties were still <3% for the green and blue continuum points, <6% for CN, and <11% for the ultraviolet continuum. For OH, the uncertainties ranged from 14% to 30%, while C 2 was 13% %, and C 3 was 48% 68%. Finally, the signal-to-noise was insufficient to provide useful results for NH in March. While not individually tabulated, the 1σ uncertainties are plotted in all figures but are usually smaller than the size of the data points.

3 1064 SCHLEICHER Vol. 138 Table 2 Photometric Fluxes and Aperture Abundances for Comet 17P/Holmes UT Date Aperture log Emission Band Flux log Continuum Flux log M(ρ) (molecule) Size log ρ (erg cm 2 s 1 ) (ergcm 2 s 1 Å 1 ) (arcsec) (km) OH NH CN C 3 C 2 UV Blue Green OH NH CN C 3 C Nov Nov Nov Nov Nov Nov Nov Nov Nov Nov Nov Nov Nov Nov Nov Nov Nov Nov Nov Nov Nov Dec Dec Dec Dec Dec Dec Dec Dec Dec Jan Mar Mar Mar PHOTOMETRIC RESULTS 3.1. Production Rates Overview The derived production rates given in Table 3 are plotted as logarithms in Figure 1 as a function of time from perihelion (2007 May 4.50). All species exhibit exponential decays in these plots (corresponding to double-exponential decays in linear units), dropping by factors between about 200 and 500 during this 125-day interval beginning on 2007 November 1 (ΔT = ). The large apparent scatter within individual nights is almost entirely due to large trends with aperture size, with larger apertures always yielding higher production rates. Dust A(θ)fρ also systematically exhibits a much larger trend with aperture size than do any of the gas species production rates. We have chosen to not adjust the A(θ)fρ values for phase angle because the phase angle effect is expected to be small (<16% variation) over the 8 range in phase angles (between 11 and 19 ) based on our derived 1P/Halley phase function (Schleicher et al. 1998), and is overwhelmed by the strong temporal and aperture trends Aperture Effects The aperture trends seen for every species, and especially the large trend for the dust, imply that one or more of the assumptions used in our standard computation of gas production rates and dust A(θ)fρ values are incorrect. Fortunately, the characteristics of these trends can be used to derive additional properties of the outburst and subsequent excess gas vaporization and dust release above baseline, i.e., pre-outburst levels. With the apparent exponential decay in log production as a function of time (Figure 1), it is clear that a major cause for the aperture trends is simply the age of the material being measured. As is always the case, smaller apertures selectively see younger material that has not had sufficient time to escape the projected column defined by the photometer entrance aperture. Larger apertures contain proportionally older material and therefore, in the case of Holmes, material released at a time of higher production rates. Plots showing the log production rates as a function of projected aperture radius are shown in the left-hand panels of Figure 2 for OH and for dust on the four nights having the largest range of aperture sizes and, conveniently, nearly equal time intervals: November 1, 11, 20, and December 3. Examining the dust first, there is approximately a factor of 4 increase in the derived A(θ)fρ values from the smallest to largest aperture (ρ from to km). Power-law fits provide excellent matches to the curvature seen in the data on each night. The case for using power-law fits is confirmed by the fact that linear leastsquares fits provide excellent matches when plotting log ρ rather than ρ as the abscissa (right-hand panels); the resulting slopes

4 No. 4, 2009 POST-OUTBURST PHOTOMETRY OF COMET 17P/HOLMES 1065 Table 3 Photometric Production Rates for Comet 17P/Holmes UT Date ΔT a log r H log ρ log Q (molecule s 1 ) log A(θ)fρ (cm) log Q (day) (AU) (km) OH NH CN C 3 C 2 UV Blue Green H 2 O 2007 Nov Nov Nov Nov Nov Nov Nov Nov Nov Nov Nov Nov Nov Nov Nov Nov Nov Nov Nov Nov Nov Dec Dec Dec Dec Dec Dec Dec Dec Dec Jan Mar Mar Mar Note. a Time from perihelion, 2007 May 4.50; the onset of the outburst occurred at about 2007 October 23.7, or ΔT = of these linear fits are given in Table 4. The slopes of the log log aperture trends are essentially identical for the latter three nights, implying that whatever is causing the strong aperture effect in A(θ)fρ is largely unchanged over this month-long interval. We will return to the possible causes of the very steep slopes later in this section. Somewhat unexpectedly, the first night s aperture trend for dust is actually slightly less steep than later nights, probably because our largest aperture size (ρ = km) at this time is nearly the size of the whole outburst envelope; had we used apertures progressively larger than the outburst envelope, the derived A(θ)fρ values would have dropped rapidly. Looking next at the aperture trends for the derived production rates of OH, similar in magnitude to those of the other four gas species, the data are again reasonably well-fit with power laws for log Q versus ρ (Figure 2, top left panel) and again with linear fits when plotting as a function of log ρ (top right panel). As in the case of the dust, larger apertures yield larger production rates, but the amount of variation with aperture size is much less than was observed for A(θ)fρ. For OH, the derived Qs from the largest aperture are about a factor of 1.6 greater than for the small aperture results. As evident from Table 4, where we list the slopes of the linear fits on each of the four nights for each species, C 3 exhibits the smallest aperture trends, presumably associated with C 3 having the shortest parent lifetime. Overall, the progressively increasing aperture trends for C 3, CN, NH, and OH each day are completely consistent with the expected phase lags in the outer coma associated with the relative lifetimes of parents (and grandparents) coupled with outflow velocities, combined with the overall exponential decay in production rates with time. However, while C 2 would be expected to have an intermediate aperture trend, it actually exhibits the largest aperture trend of all of the gas species. This is apparently associated with the fact that C 2 spatial radial profiles are generally flatter near the center of a coma than can be fit with the Haser model, and that small trends in derived production rate as a function of aperture size are seen in C 2 when no trends are observed for the other species. Examples from the Lowell comet database include Comets 1P/Halley, Hyakutake (1996 B2), and 153P/Ikeya-Zang (2002 C1). Therefore, the steeper aperture trend seen for C 2 here in the case of Comet Holmes appears to simply be a combination of normal aperture trends for C 2 along with an intermediate lifetime for its parents and grandparents. Because the amount of excess gas in the outer coma is consistent with the inner coma gas production only 1/2 to 2 days previously, i.e., consistent with the effective parent lifetime of each species coupled with nominal gas velocities for daughter species of 1 kms 1, we conclude that there is

5 1066 SCHLEICHER Vol. 138 Nov 1 Dec 1 Jan 1 Feb 1 Mar 1 Nov 1 Dec 1 Jan 1 Feb 1 Mar 1 30 OH 28 CN NH 28 C log A(θ)fρ [cm] ΔT [day] Green Cont ΔT [day] Figure 1. Logarithmic production rates for Comet 17P/Holmes as a function of time (ΔT) from perihelion (2007 May 4.50). Our results are plotted for each species, along with 1σ uncertainties. The estimated time of onset of the outburst (2007 October 23.7) is indicated with the vertical dotted line. The large apparent scatter on each night is primarily due to a large trend in derived production rates as a function of aperture size, which in turn is associated with the extreme, double-exponential decrease in linear production rates as a function of time. In other words, larger aperture sizes contain a greater proportion of older material and, because production rates are decreasing extremely rapidly, the larger apertures reflect a time of higher production. Table 4 Aperture Size Trends for Comet 17P/Holmes Species Slopes of Linear Fits to Aperture Trends a Mean Aperture Nov 1 Nov 11 Nov 20 Dec 3 Trend Slope b OH 0.20 ± ± ± ± NH 0.25 ± ± ± ±.04 CN 0.26 ± ± ± ±.03 C ± ± ± ±.06 C ± ± ± ±.05 Green Cont ± ± ± ± Notes. a Aperture trend slopes of linear fits are for log Q or log A(θ)fρ as a function of log ρ. b Mean values are calculated for the primary species, water and dust, which do not show clear trends with time. C 3 no evidence that an extended source of gas is needed to explain these aperture trends, and in fact a significant extended source would create much stronger aperture trends than we see. This, in turn, implies that the source of the ongoing gas production weeks and even months after the outburst must be from either the nucleus or near-nucleus regime. In comparison to the gas, the dust ejecta plume from the outburst was seen to expand at a near constant projected velocity of about 0.2 km s 1 for several weeks (Montalto et al. 2008), considerably slower than the nominal gas velocities. For a given projected aperture size, dust is therefore expected to remain in the aperture proportionally longer, and since this older dust was

6 No. 4, 2009 POST-OUTBURST PHOTOMETRY OF COMET 17P/HOLMES OH OH Green Cont. Green Cont. 5.5 log A(θ)fρ [cm] ρ [km] log ρ [km] Figure 2. Logarithmic production rates for OH and dust as a function of linear (left) and logarithmic (right) projected aperture radius for the four nights having the largest ranges of aperture size, from top to bottom: November 1, 11, 20, and December 3. Exponential fits are shown for the linear ρ plots and linear fits are shown for log ρ plots. The slopes of the linear fits for these log log plots and those of the minor species (not plotted) are listed in Table 4. The vertical dotted line corresponds to a projected aperture size of km, a size at which we extract near-instantaneous production rates (see the text for details). released when production rates were even higher, this potentially explains the larger aperture trends observed for the dust as compared to any of the gas species. An additional complication is that radiation pressure will push dust grains into the tail; this, combined with the relatively small phase angles (11 19 ) during our observations, means that a significant portion of the tail contaminates the larger apertures. Determining the relative importance of these effects and developing an understanding of how much of the observed dust in the months following the outburst was released at the time of the outburst and how much was released during subsequent outgassing will require detailed modeling of the dust morphology and is beyond the scope of this paper. We, instead, can provide a practical constraint for the subsequent on-going dust (and gas) production by extrapolating to a small aperture using the measured aperture trends discussed above. For instance, dust having a projected velocity of 0.1 km s 1 would remain in a small projected aperture radius of km for about 1 day, and even dust having a velocity of 0.01 km s 1 would only remain in the aperture for about 1 week. Such a small aperture also effectively excludes the dust tail. The higher velocities and finite lifetimes of the observed gas species mean that the vast majority of the gas observed within such a small aperture must have been released no more than a day or two prior to the observation. Therefore, by extrapolating the aperture trends to an aperture size of km, we can provide an effective measure of near-instantaneous production rates of Comet Holmes Aperture Adjusted Production Rates The resulting small aperture (ρ = km) extrapolations for log Q(OH) and A(θ)fρ as a function of time are presented in Figure 3. Because there are no clear variations in the aperture trends over the November 1 to December 3 interval, we use the mean slopes of the linear fits to the aperture trend from these nights (Table 4; from Figure 2) to adjust all OH (i.e., water) and dust measurements. The values on our first night of 2007 November 1 (ΔT = day) for OH and dust are molecules s 1 and cm, respectively, and decrease to molecules s 1 and cm, respectively, on the combined 2008 March 4 and 5 (ΔT = and day). As noted in Section 2, we can apply our empirical adjustment to the OH (Haser) production rates to yield H 2 O vectorial equivalent values. Since this conversion incorporates an r 0.5 H dependence of the water outflow velocity in addition to a branching ratio of 0.90 and a value of r H at which the two models yield the same production rate, the resulting water

7 1068 SCHLEICHER Vol. 138 log A(θ)fρ [cm] Nov 1 Dec 1 Jan 1 Feb 1 Mar ΔT [day] Green Cont. 280 Figure 3. Logarithmic production rates for OH and dust as a function of time following the adjustment for aperture size by extrapolation to a uniform, small aperture radius of km based on the mean slopes of the linear fits in the righthand plots of Figure 2. The time of onset of the outburst (2007 October 23.7) is indicated with the vertical dotted line; peak brightness was reached 24 hr following the onset. The dashed curves are the respective best exponential fits to all of the observations, which are also extrapolated backward in time to October 25.2, the middle of the first full day following the end of the outburst. The full range of decreasing production rates is now slightly smaller for the dust than for OH, but the dust curve is somewhat steeper than OH in the days shortly after the outburst. The overall similarity of the curves implies a nearly constant dust-to-gas ratio with time. production rate can actually be smaller than the OH (Haser) production rate at large heliocentric distances such as in the case of Comet Holmes. Indeed, for the heliocentric distance range in question ( AU), the resulting water values are actually slightly less than the OH Haser values, by between factors of and 0.786, respectively, yielding water values of molecules s 1 and molecules s 1 for the start and end of our interval of observations. Overall, the nearinstantaneous rates for OH (water) decreased by about a factor of 340 (350) during this interval, while the dust decreased by a factor of 220. Even though the amount of decrease is slightly larger for the gas than for the dust, the dust actually exhibits a somewhat steeper decline early in the time interval. These best estimates of the near-instantaneous logarithmic gas and dust production rates for each night of observation just described and shown in Figure 3 can themselves be reasonably well fit by exponentials (dashed curves), thereby permitting us to extrapolate backward in time closer to the time of the outburst, even if the cause of this exponential fall-off in the log of the production rates as a function of time is unknown. For OH, the equation of the fit is: log Q(OH) = exp( ( ΔT)), while for dust the equation is: A(θ)fρ = exp( ( ΔT)). Evidence strongly suggests that the brightness of Comet Holmes reached a maximum about 6 8 hr prior to October 25 (Yoshida 2008; Sekanina 2008). Extrapolation to the middle of the first full day after reaching peak brightness (October OH 25.2 or ΔT = ) yields Q(OH) = molecules s 1 and Q(H 2 O) = molecules s 1 ; this is probably a good estimate for new, on-going activity, because of the short effective lifetime of OH within this small aperture, as was discussed above. In fact, our extrapolated water value for October of molecules s 1 is in very good agreement with the measured value of 4.5 ± molecules s 1 by Dello Russo et al. (2008). However, any water vapor released during the outburst itself and largely dissociated into H and O atoms by November 1 would be in addition to this extrapolated value. This may explain the higher water production rates derived by Combi et al. (2007) of molecules s 1 for October, since they measure the entire hydrogen coma with the SWAN camera on Solar and Heliospheric Observatory (SOHO). In addition, Combi et al. measure a much more gradual decline in production rates than do we by the middle of November, obtaining a value of molecules s 1, whereas our near-instantaneous value is molecules s 1 a factor of 13 smaller. Given that we observe steep declines in production rates for all species at each aperture size in a range of aperture radii of km centered on the nucleus, we suspect that the much more gradual decline measured with SWAN is caused by the vaporization of icy grains in the now very extended cloud of debris from the original outburst. As noted later, the debris cloud separates from the nucleus with time and so would have only a small effect on our photometric measurements. More uncertain is the rate of on-going dust release following the outburst. Our exponential fit suggests a peak, small-aperture A(θ)fρ = cm on October 25, but A(θ)fρ is only a proxy for dust production and includes several assumptions complicating the interpretation of a result in circumstances such as the immediate aftermath of an extreme outburst. In fact, such assumptions, such as an unchanging outflow velocity of the dust, may be incorrect and could be the cause of the steeper decay for the dust than for the gas as mentioned in the previous paragraph. In spite of these complications, the quantity A(θ)fρ may be more indicative of the ongoing dust production than would be expected. Because the exponential fits for OH and for dust generally have similar shapes, it is reasonable to assume that the intrinsic dirt-to-ice ratio of the vaporizing ice was constant throughout the post-outburst interval. Based on this observed characteristic, we suggest that the same mixture existed in the material released during the actual outburst as we measured during the subsequent weeks and months Dust-to-Gas Ratio, Water Vaporization, and Total Gas and Dust Released After adjusting to a small aperture size for near-instantaneous production rates, we can derive a dust-to-gas ratio using our standard technique of computing the ratio of A(θ)fρ to Q(OH). The overall mean is cm s molecule 1 ; however, the November 1 and the March points have higher values ( and cm s molecule 1, respectively) than all of the intermediate nights which have a mean value of cm s molecule 1. In any case, the entire range of values for Holmes dust-to-gas ratio is at the high end of the twoorder-of-magnitude spread for other comets in our database (compare A Hearn et al. 1995); post-outburst Holmes is similar to Comet Hale Bopp s very large dust-to-gas ratio (Schleicher et al. 1997). This high value for Holmes is somewhat less surprising when one factors in its relatively large perihelion distance of 2.05 AU. As shown by A Hearn et al., comets

8 No. 4, 2009 POST-OUTBURST PHOTOMETRY OF COMET 17P/HOLMES 1069 exhibit a strong trend in their measured dust-to-gas ratio as a function of perihelion distance, with larger perihelion distance comets having higher dust-to-gas ratios believed to be associated with less thermal processing of their nucleus surfaces. Even so, Holmes is still about five times dustier than other comets with similar perihelia. It is unclear to what extent, if any, that the contribution of the on-going release of icy grains discussed next affects our measured dust-to-gas ratio. By combining our measured production rates with a water ice vaporization model, we can estimate the total surface area of ice that must have been exposed to sunlight as a function of time. Since the vaporization model depends on several unknown details such as the size of the ice grains, chunks, boulders, etc released in the outburst, we have chosen to simply use the same standard water vaporization model (based on Cowan and A Hearn 1979) as we have in the past (compare A Hearn et al. 1995) to provide an inferred effective cross-section of ice to sunlight. In round numbers, the surface area of ice was about 500 km 2 on November 1, 63 km 2 on November 20, 13 km 2 on January 1, and 3 km 2 on March 4/5. Our previous extrapolation to the first day following the outburst, October 25, is 1200 km 2. These results can be compared to the size of the nucleus of Comet Holmes, where an effective radius of 1.71 km found by Lamy et al. (2000) corresponds to a surface area of 37 km 2. Because the near-instantaneous water production rate during the first month post-outburst is much larger than can be explained even if the entire nucleus surface was exposed ice, a majority contribution must have been from an extended source of icy grains or chunks. However, the previous findings regarding aperture trends ruled out the possibility of a large extended source of ice particles centered on the nucleus (as distinguished from a possible very widely distributed debris cloud from the outburst itself). The only solution to this apparent contradiction is that a substantial cloud of icy particles must have remained in the innermost coma (< km), possibly hidden within the seeing disk, or, perhaps more likely, been continually replenished over time. We can also integrate the water production for our observing interval, to derive a total amount of ice that was vaporized. From November 1 to March 5, the total amount of water was about molecules, corresponding to a mass of kg. Extrapolating back to the outburst roughly adds an additional water molecules, for a total of kg, which corresponds to a volume of m 3 or 0.02 km 3 assuming a density of unity. Again using the nucleus radius determined by Lamy et al. (2000), the corresponding nucleus volume is 21 km 3. Therefore, approximately 0.1% of the nucleus vaporized in just this four and one-half month interval; if the bulk water ice density in the nucleus is smaller, such as 0.5 gm cm 3 often assumed, then the fraction of the nucleus vaporized was a factor of 2 higher, i.e., 0.2%. Again, these are lower limits, as they do not include the unknown amount of ice initially vaporized during the day of the outburst. Because the conversion of dust A(θ)fρ values to a number of dust grains or a total mass released per unit time is strongly dependent on the unknown particle size distribution, we usually do not attempt to take our dust measurements any further. However, in the interest of estimating a total mass loss for Comet Holmes, we have applied the conversion factor estimate by C. Arpigny (1994, private communication; also see A Hearn et al. 1995) that an A(θ)fρ value of 1000 cm corresponds to a dust production of 1 metric ton s 1, i.e., 1 cm for A(θ)fρ corresponds to1kgs 1. The integral of A(θ)fρ from November 1 to March 5 yields an approximate total of kg. Because the exponential function for dust is initially steeper than for the gas, the dust released in the 7 days prior to November 1 is proportionally greater than after November 1, with an additional kg, for a total of kg. Note that this means that the total mass of dust production was about a factor of 10 greater than the mass of water ice that was vaporized. Assuming a density of unity, this implies a volume of 0.2 km 3 or, at one-half this density, 0.4 km 3, corresponding to 1% 2% of the nucleus volume. In conclusion, we estimate that 1% 2% of the nucleus was released and subsequently vaporized (ice) or dispersed (dust). In the case of the refractory component, this is likely to be a lower limit, as most of the mass may remain in larger chunks, boulders, etc., which are essentially undetectable at visible wavelengths. We note that our estimate is in good agreement with the estimates by Sekanina (2008) and Altenhoff et al. (2009) Production Rate Ratios The relative abundances of C 2 -to-cn places Comet Holmes near the lower end of the typical composition class as defined by A Hearn et al. (1995) and currently being updated (compare Schleicher et al. 2007). This classification of typical for Holmes holds whether or not one adjusts for aperture trends. Moreover, with the typical class showing a slight trend with perihelion distance, probably associated with the general trends in derived C 2 production rates with aperture size discussed above, Holmes C 2 -to-cn ratio is in the mid-range of objects having typical composition with perihelia greater than 2 AU. The only abundance ratio we can compare to other researchers is that for CN-to-OH, for which we measured a value of 0.48% ± 0.02% on November 1, while Dello Russo et al. (2008) measured a value of HCN-to-water of 0.54% ±.07% on October. The excellent agreement strongly implies that HCN is the primary parent of CN in Comet Holmes Dust Color To within the uncertainties, no trends over time are evident in the color of the dust grains; the average delta log A(θ)fρ between the green and UV continuum filters is only +0.11, corresponding to a moderate reddening of 16% ± 3% per 1000 A. As usual, the color is slightly redder in the near-uv (20% ± 4% for blue vs. UV continuum points), and less red in the visible (10% ± 3% for green versus blue). Finally, we note a consistent but very small color trend with aperture size, with smaller apertures always having slightly redder colors. 4. DISCUSSION AND SUMMARY 4.1. Primary Findings Narrowband filter photometry was obtained of Comet 17P/ Holmes on a total of nine nights over a 125 day interval beginning 2007 November 1, only 8 days following its extreme outburst. Our basic derived gas and dust production rates using our standard methodology exhibited exponential decays when plotting the logarithm of production rates as a function of time, with each species dropping by factors of about between November 1 and March 5. Even at the end of our observations, gas and dust production rates remained far above pre-outburst levels. All gas species exhibited clear trends with aperture size, with larger apertures yielding higher production rates, throughout

9 1070 SCHLEICHER Vol. 138 the observing interval. As is always the case in comets, larger aperture sizes contain a higher proportion of older material; in the case of Holmes post-outburst, older material was released at a time of higher production than for newer material. Moreover, the size of the aperture trend varied among species in a manner consistent with differing parent and daughter lifetimes, with shorter-lived species such as C 3 exhibiting the smallest aperture trend. Finally, the difference in measured small versus large aperture production rates was consistent with the steep rate of drop-off in production rates with time. When combined, these and other facts directly imply several results. First, the unusual aperture trends observed for all of the gas species were simply due to the strong temporal drop-off in production rates. Second, on-going gas production continued throughout the timeframe of our observations, thus a much larger supply of volatiles remained exposed to sunlight at the end of this interval than prior to the outburst. Third, the water production rates we measured during the first month far exceeded that possible even if the entire surface of the nucleus was active, thus a large volume of ice grains/chunks/boulders must have been released into space. Fourth, because the observed radial distributions of each gas species were not consistent with a significant distributed source centered on the nucleus, the ice particles must have remained within 10 4 km of the nucleus or, perhaps more likely, have been continually re-supplied from the nucleus at a progressively decreasing rate. The dust production, based on our standard proxy A(θ)fρ, exhibited trends with aperture size having the same sense as the gas but a much larger slope (see Figure 2). One clear cause is simply due to the typically slower outflow velocities of dust as compared to gas, resulting in older dust remaining in the aperture longer than do gas molecules. Another cause is that grains generally live nearly forever, unlike gas molecules, although ice grains will eventually vaporize. Therefore, it is plausible that the steeper aperture trend in A(θ)fρ is simply due to this combination of causes for older dust to remain in the apertures much longer than does the gas. The case for this explanation is strengthened by the very fact that the long-term temporal decay curves for both the dust and the gas are very similar; having the same overall decline with time strongly suggests a common source for both, and since the gas production requires a continuing production, we can infer that not all of the dust was released during the initial outburst but rather also was released throughout the observed post-outburst interval. By extrapolating our multi-aperture production rates to a fixed, relatively small aperture (ρ = km), we have computed near-instantaneous production rates and, because these resulting values are also well-fit with exponential curves when plotted as log Q or log A(θ)fρ versus time, we can compute production rates throughout the observing interval. Furthermore, by extrapolating these curves backward in time, we can calculate useful estimates of the production rate for the days immediately following the outburst. This process yields a peak water production rate of molecules s 1 and an A(θ)fρ of cm. To attain this rate of ice vaporization implies a peak effective surface area of 1200 km 2, corresponding to more than 30 times the total surface of the nucleus. The integrated water production through March 5 implies that a mass of water ice of kg corresponding to a volume of 0.02 km 3 was vaporized, or about 0.2% of the nucleus volume assuming a bulk density of 0.5 gm cm 3. A simple, rough conversion factor for estimating dust mass from A(θ)fρ yields a bulk dust mass of roughly kg or a factor of 10 larger mass than for the water ice. Assuming a bulk density of implies that at least 1% 2% of the nucleus was released as dust during 4 months of subsequent activity; it is unknown how much additional dust was released nor how much additional ice was vaporized during the actual outburst on October 23 and Additional Implications The very high water production rates measured by us during the initial 4 6 weeks after the outburst require a dominant icy grain component in the inner coma to supply the necessary cross-section to solar heating. While this could in principle be produced by a substantial cloud of icy particles in orbit about the nucleus, it seems more likely that this inner-coma population of icy grains (or larger chunks) was continually being replenished by material escaping the surface. A continuous but declining supply of solid volatiles and nonvolatiles escaping into the coma would also help explain why the total brightness of Holmes decreased extremely slowly even though gas was dissociating and dust was dispersing from the system. It remains unclear whether the observed double-exponential decay in linear production rates as a function of time is a direct reflection of the rapid decrease in the rate of release of ice and dust grains from the nucleus or instead is a combination of a somewhat less steep decrease in the release of particles coupled with the rate at which the particles vaporize away, which in turn would strongly depend on the unknown particle size distribution. In addition, the much higher water production rates determined by Combi et al. (2007) from SWAN measurements of the entire comet in mid-november probably imply an extensive icy particle component associated with the original outburst but largely separate from the near-nucleus water production we measured. In fact, there may well have been a total of four components to the total water production of Holmes. First, vaporized water released directly from the nucleus during the initial 24 hr outburst interval; this component would be dissipated by the time of the onset of our observations. Second, an expanding icy grain halo released during the outburst and vaporizing away with time, with both the expansion rate and the rate at which grains are lost dependent on the unknown icy particle size distribution; this component would likely be too dispersed to be detected by us. Third, a near-nucleus supply of icy grains, presumably continually being resupplied from the new active region on the nucleus created from the outburst; this component dominates our November observations. Fourth, water vaporizing directly from the new active region; this component might dominate by the time of our March observations and, in the case of Holmes behavior following the 1892 outburst, probably explains the higher activity in 1899 as compared to subsequent apparitions An Outburst Scenario Several possible scenarios have been presented to explain either the cause of Holmes extreme outburst and/or the early subsequent evolution of coma morphology. Generally, the outburst is considered to be the result of a subsurface buildup of gas pressure, presumably due to slow, long-term heat infiltration and an associated slow increase in pressure from water ice sublimation or a subsequent phase change in the ice (compare Sekanina 2008; Altenhoff et al. 2009). We speculate that an alternate source of a buildup in pressure could have taken place sublimation of a more volatile ice such as CO or CO 2 at a lower temperature than required for water ice. Our previous