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CYANOGEN JETS and the ROTATION STATE of COMET MACHHOLZ (C/2004 Q2) Tony L

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CYANOGEN JETS and the ROTATION STATE of COMET MACHHOLZ (C/2004 Q2) Tony L The Astronomical Journal, 133:2001Y2007, 2007 May # 2007. The American Astronomical Society. All rights reserved. Printed in U.S.A. CYANOGEN JETS AND THE ROTATION STATE OF COMET MACHHOLZ (C/2004 Q2) Tony L. Farnham1 Department of Astronomy, University of Maryland, College Park, MD 20742, USA; [email protected] Nalin H. Samarasinha2 National Optical Astronomy Observatory, Tucson, AZ 85719, USA; and Planetary Science Institute, Tucson, AZ 85719, USA Be´atrice E. A. Mueller1 Planetary Science Institute, Tucson, AZ 85719, USA and Matthew M. Knight1 Department of Astronomy, University of Maryland, College Park, MD 20742, USA Received 2006 June 16; accepted 2007 January 25 ABSTRACT Extensive observations of Comet Machholz (C/2004 Q2) from 2005 February, March, and April were used to derive a number of the properties of the comet’s nucleus. Images were obtained using narrowband comet filters to isolate the CN morphology. The images revealed two jets that pointed in roughly opposite directions relative to the nucleus and changed on hourly timescales. The morphology repeated itself in a periodic manner, and this fact was used to determine a rotation period for the nucleus of 17:60 Æ 0:05 hr. The morphology was also used to estimate a pole orientation of R:A: ¼ 50,decl: ¼þ35, and the jet source locations were found to be on opposite hemispheres at mid- latitudes. The longitudes are also about 180 apart, although this is not well constrained. The CN features were mea- sured to be moving at about 0.8 km sÀ1, which is close to the canonical value typically quoted for gas outflow. Future modeling of the CN features will be used to improve and extend these results. Key words: comets: general — comets: individual (Machholz C/2004 Q2) 1. INTRODUCTION 2005), and thermal models of the nucleus rely on the ability to distinguish between rapid rotators and slow rotators (e.g., Weissman Comet Machholz (C/2004 Q2) was discovered by D. E. & Kieffer 1981). Machholz on 2004 August 27 (Machholz et al. 2004). Soon after During an observing campaign of comet Tempel 1, we also ob- discovery, it was predicted to become very bright before its peri- tained images of comet Machholz at roughly monthly intervals helion passage on 2005 January 24. It lived up to expectations, between 2005 January and June. In mid-January only broadband reaching naked-eye brightness at about magnitude 3.5 around images were obtained, but they showed little structure, in contrast the time of its closest approach to Earth on January 5 (0.35 AU), to the observations of Sastri et al. (2005a). Starting in February, we but by 2005 April it was fading rapidly as both heliocentric and geocentric distances increased.3 On 2005 February 9, a rotation obtained images using the HB narrowband comet filters (Farnham et al. 2000) to isolate the spatial structure of the gas species and period and a pole orientation were announced by Sastri et al. dust in the coma. These images showed strong cyanogen (CN) (2005a, 2005b). Their solution was based on modeling of features observed in the dust coma in early and mid-January. They found jets and features that changed on a timescale of hours. These changes were caused by the rotation of the nucleus. After the a rotation period of 9:1 Æ 2 hr and a pole at right ascension of discovery of these jets in February, a concentrated effort was 190 Æ 10 and declination of +50 Æ 10. From their models, undertaken in March and April to obtain more complete obser- they also found three jets in the southern hemisphere, at latitudes vations, which could be used to derive the rotation state of the in the ranges of 0 to À15 , À35 to À50 , and À70 to À78 . nucleus. An accurate measurement of the rotational properties of a comet’s nucleus is important because the adopted rotation state 2. OBSERVATIONS can have a significant impact on the interpretation of other obser- vations. For example, the radar albedo and nucleus radius based 2.1. Observations and Data Reduction on radar observations are strongly dependent on the value used A list of the observing conditions and geometries is given in for the rotation period and the spin axis orientation (Nolan et al. Table 1. The primary observations that are discussed here were obtained 2005 March 11Y14 and 2005 April 9Y14 at the Kitt Peak National Observatory (KPNO) 2.1 m telescope using the F3KB 1 Visiting Astronomer, Kitt Peak National Observatory, National Optical As- CCD (which has 0.1900 pixelÀ1). On these nights, the comet was at tronomy Observatory, which is operated by the Association of Universities for Research in Astronomy (AURA), Inc., under cooperative agreement with the Na- high northern declinations (84 in March and 71 in April) tional Science Foundation. and thus was observable for about 10 hr per night. These windows 2 Kitt Peak National Observatory, National Optical Astronomy Observatory, of visibility provided uninterrupted coverage of long spans of the which is operated by AURA, Inc., under cooperative agreement with the National Science Foundation. comet’s rotation, helping to reduce the chances of aliasing of the 3 These data were taken from a Web page maintained by S. Yoshida, http:// rotational phase from night to night. The observing and geometric www.aerith.net/obs/comet.html. The data used here were accessed 2006 May 9. conditions were excellent, with typical seeing around 100 and pixel 2001 2002 FARNHAM ET AL. Vol. 133 TABLE 1 Observing Conditions Solar Phase KPNO rH Á Angle UT Date Telescope (AU) (AU) (deg) 2005 Jan 16Y17 ......... 4.0 m 1.21 0.38 45Y46 2005 Feb 3Y4 ............ 2.1 m 1.21Y1.22 0.52Y0.53 52 2005 Mar 11Y14 ........ 2.1 m 1.39Y1.42 0.88Y0.91 45Y44 2005 Apr 9Y14 .......... 2.1 m 1.65Y1.70 1.18Y1.24 37Y36 2005 May 14Y17 ....... 2.1 m 2.02Y2.05 1.61Y1.66 30Y29 2005 Jun 4Y7............. 2.1 m 2.25Y2.28 1.94Y1.99 27Y26 Fig. 1.—CN images of comet Machholz from February 3.169 and 4.272, showing the structure that prompted further follow-up to determine the rotation scales of 125 and 165 km at the comet in March and April, period. The nucleus is at the center of each image, with two jets extending out- ward in opposite directions. The images have been enhanced by dividing out the respectively. azimuthally averaged radial profile as discussed in the text. (With this processing, High-resolution observations were also obtained on Feb- circular portions of the image are extracted, leaving the corners black.) The ruary 3Y4, with a pixel scale of 70 km, although only a few February 3 image is 3:9 ; 104 km across, and the February 4 image is 5 ; 104 km images were taken. Additional observations were obtained on across; north is up, and east is to the left. The projected sunward direction is marked. May14Y17 and June 4Y7, but by this time the comet’s heliocen- tric and geocentric distances were increasing, and the structures evaluate whether the features are real or if artifacts have been in- were affected by a low signal-to-noise ratio (S/N). For the current troduced. For the images examined here, we find that division by work, the May and June observations will not be addressed. a simple azimuthal average works well to show the CN mor- Images were obtained using a Harris broadband R filter and phology. This technique computes the mean value of all pixels at the HB narrowband comet filters (Farnham et al. 2000), which a given distance from the optocenter to generate an average ra- isolate the spatial structure of the gas (CN, OH, and C ) and dust 2 dial profile of the coma. Each pixel is then divided by the average in the coma. The discussion presented here concentrates on the value for its radial distance to improve the contrast of the features. results from the CN filter, with other filters to be discussed in a One advantage of this technique is that it uses the image itself to future paper. The data were reduced using standard procedures. remove the central peak and the radial falloff, requiring no a priori The bias was removed using a combination of the overscan re- assumptions about the coma shape. It also preserves any azi- gion and a master bias frame. Flat-fielding was performed using muthal asymmetries that may exist. This enhancement reveals CN dome flats to remove pixel-to-pixel variations across the CCD. jets that are reminiscent of those seen in comet Halley (A’Hearn Asymmetries in the coma are visible in the reduced CN images et al. 1986). Figure 1 shows two images from February and illus- from February and March, even with no image enhancements. trates how the enhancement reveals the CN features. The differ- These asymmetries indicate that there are features present in the ence in morphology on the two days prompted us to obtain follow coma, likely produced by isolated active areas on the surface. up observations in subsequent months. (Note that all images are Coma features of this kind are not uncommon, having been ob- shown with north up and east to the left unless otherwise noted.) served in comets Halley (e.g., A’Hearn et al. 1986), Hyakutake (e.g., Schleicher & Woodney 2003; Samarasinha et al. 2004), and Hale-Bopp (e.g., Farnham et al. 1998; Lederer et al. 1998; 3. COMA MORPHOLOGY Mueller et al. 1997), among others, and can be used to infer prop- After the CN images from February, March, and April have erties of the nucleus.
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