WISE/NEOWISE observations of Active Bodies in the Main Belt
James M. Bauer, A. K. Mainzer, Tommy Grav, Russell G. Walker, Joseph R. Masiero, Erin K. Blauvelt, Robert S. McMillan, Yan R. Fernández, Karen J. Meech, Carey M. Lisse, Roc M. Cutri, John W. Dailey, David J. Tholen, Timm Riesen, Laurie Urban, Alain Khayat, George Pearman, James V. Scotti, Emily Kramer, De'Andre Cherry, Thomas Gautier, Stephanie Gomillion, Jessica Watkins, Edward L. Wright, WISE Team
AAMBOs, Bauer et al.
WISE/NEOWISE observations of Active Bodies in the Main Belt
James M. Bauer , A. K. Mainzer , Tommy Grav , Russell G. Walker , Joseph R. Masiero , Erin K. Blauvelt , Robert S. McMillan , Yan R. Fernández , Karen J. Meech , Carey M. Lisse , Roc M. Cutri , John W. Dailey , David J. Tholen , Timm Riesen , Laurie Urban , Alain Khayat , George Pearman , James V. Scotti , Emily Kramer , De’Andre Cherry ,Thomas Gautier , Stephanie Gomillion , Jessica Watkins , Edward L. Wright , and the WISE Team Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Drive, MS 183-‐401, Pasadena, CA 91109 (email: [email protected]) Infrared Processing and Analysis Center, California Institute of Technology, Pasadena, CA 91125 Planetary Science Institute, 1700 East Fort Lowell, Suite 106, Tucson, AZ 85719-‐2395 Monterey Institute for Research in Astronomy, 200 Eighth Street, Marina, CA 93933 Lunar and Planetary Laboratory, University of Arizona, 1629 East University Blvd., Kuiper Space Science Bldg. Department of Physics, University of Central Florida, 4000 Central Florida Blvd., P.S. Building, Orlando, FL 32816-‐2385 Institute for Astronomy, University of Hawaii, 2680 Woodlawn Dr., Manoa, HI 96822 NASA Astrobiology Institute, University of Hawaii, Manoa, HI 96822 Applied Physics Laboratory, Johns Hopkins University, 11100 Johns Hopkins Road Laurel, MD 20723-‐6099 Department of Physics and Astronomy, University of California, PO Box 91547, Los Angeles, CA 90095-‐1547
Short Title: WISE IR observations of
AMBO s. MBOs, Bauer et al.
Abstract
We report results based on mid-infrared photometry of 5 active main belt objects (AMBOs) detected by the Wide-field Infrared Survey Explorer (WISE) spacecraft. Four of these bodies, P/2010 R2 (La Sagra), 133P/Elst-Pizarro, (596) Scheila, and 176P/LINEAR, showed no signs of activity at the time of the observations, allowing the WISE detections to place firm constraints on their diameters and albedos. Geometric albedos were in the range of a few percent, and on the order of other measured comet nuclei. P/2010 A2 was observed on April 2-3, 2010, three months after its peak activity. Photometry of the coma at 12 and 22 µ m combined with ground-based visible-wavelength measurements provides constraints on the dust particle mass distribution (PMD), dlogn/dlogm, yielding power-law slope values of α = -0.5 +/- 0.1. This PMD is considerably more shallow than that found for other comets, in particular inbound particle fluence during the Stardust encounter of comet 81P/Wild 2. It is similar to the PMD seen for 9P/Tempel 1 in the immediate aftermath of the Deep Impact experiment. Upper limits for CO & CO production are also provided for each AMBO and compared with revised production numbers for WISE observations of 103P/Hartley 2. Introduction
Until recently no main belt asteroids were ever seen to exhibit dust ejection or cometary activity. However, in 1996 the discovery of activity from 133P/Elst-Pizarro (Elst et al. et al.
MBOs, Bauer et al. perihelion (cf. Hsieh et al 2010). As of late 2011, 7 bodies in the main belt have been identified as main belt comets (i.e. asteroids with associated dust tails, henceforth referred to as active main belt objects, or AMBOs). Their activity and optical qualities have been well-studied (cf. Hsieh et al. et al. et al. production are also presented and compared with recomputed values of CO production for 103P/Hartley 2 that supersede previously published rates based on WISE fluxes. The WISE mission surveyed the sky at four mid-IR wavelengths simultaneously, 3.4 µ m (W1), 4.6 µ m (W2), 12 µ m (W3) and 22 µ m (W4), with approximately one hundred times improved sensitivity over the Infrared Astronomical Satellite (IRAS) mission (Wright et al. et al. et al. MBOs, Bauer et al. including Near-Earth Objects (NEOs), main belt asteroids, comets, Trojans, and Centaurs. By the end of the spacecraft mission, NEOWISE identified more than 157,000 small bodies, including 123 comets (Mainzer et al. et al. et al.
Observations & Analysis
The WISE spacecraft surveyed the sky as its terminator-following geocentric polar orbit progressed about 1 degree of ecliptic longitude per day. Regular survey operations commenced on 2010 Jan. 14 (MJD 55210), imaging the sky simultaneously in all four bands until the cryogen was depleted in the secondary tank on 2010 Aug 5 (MJD 55413). The survey then entered a three-band (W1-W3) phase that lasted through 2010 Sep 30
MBOs, Bauer et al. (MJD 55469). The final phase, the post-cryogenic mission with W1 and W2, lasted from 2010 Oct 1 through 2011 Jan 31, (MJD 55592; cf. Cutri et al. et al. et al. et al.
MBOs, Bauer et al.
Table 1: Mid-IR Observations of Known Active Main Belt Asteroids
Object a N a R a (AU) Δ a (AU) α a (°) Coma? a Stacked? a Comments a (596) Scheila 10 3.93 3.14 16.8 No No -‐-‐ MJD a Start Times: 55242.35929406, 55242.49159826, 55242.62390243, 55242.69011816, 55242.82242237, 55242.95472657, 55242.95485389, 55243.08715810, 55243.21946226, 55243.35176641
MJD Start Times: 55271.99996748, 55272.13239883, 55272.26470277, 55272.39700681, 55272.46309514, 55272.46322245, 55272.52931076, 55272.59552641, 55272.66161475, 55272.72783039, 55272.79391878, 55272.86013443, 55272.99243847, 55273.12474251
P/2010 A2 16 2.07 1.73 28.8 Yes Yes -‐-‐
MJD Start Times: 55288.27578843, 55288.40809252, 55288.54039657, 55288.67270062, 55288.80500469, 55288.87122037, 55288.93730874, 55289.00352447, 55289.06961284, 55289.13582852, 55289.20191689, 55289.26813252, 55289.40043657, 55289.53274063, 55289.66504468, 55289.79734873
MJD Start Times: 55309.28058017, 55309.41288442, 55309.54518866, 55309.67749290, 55309.67762031, 55309.80979715, 55309.80992451, 55309.87601297, 55309.94210145, 55309.94222876, 55310.00831722, 55310.07453300, 55310.14062147, 55310.27292571, 55310.40523000, 55310.53753425
P/2010 R2 19 2.62 2.40 22.8 Yes Yes -‐-‐
MJD Start Times: 55356.64184157, 55356.77414537, 55356.90644912, 55357.03875296, 55357.17092939, 55357.17105676, 55357.23714494, 55357.30323309, 55357.36944865, 55357.43553689, 55357.50175322, 55357.56784223, 55357.63405781, 55357.76636159, 55357.89853814, 55357.89866545, 55358.03084197, 55358.03096933, 55358.16314579 (596) Scheila 14 3.15 2.98 18.3 No No -‐-‐
MJD Start Times: 55510.88545743, 55511.01763413, 55511.14993825, 55511.34833072, 55511.41441912, 55511.41454644, 55511.48063478, 55511.54672802, 55511.61281640, 55511.61294377, 55511.67903215, 55511.87742472, 55512.00972890, 55512.14190566
No Detection
MJD Start Times: 55320.52708031, 55320.65938453, 55320.79168874, 55320.92399296, 55321.05629718, 55321.12251292, 55321.18860130, 55321.25481710, 55321.32090552, 55321.38712131, 55321.45320973, 55321.51942547, 55321.65172964, 55321.78403387, 55323.57007663, 55323.57020395, 55323.70238080, 55323.70250817, 55323.83468502, 55323.83481234, 55323.96711712, 55324.09942130, 55324.16550962, 55324.23172537, 55324.29781380, 55324.36402954, 55324.43011796, 55324.49633370, 55324.56242208, 55324.69472630, 55324.82703043, 55324.95933464
P/2008 R1 13 3.46 3.27 16.6 -‐-‐ -‐-‐
No Detection
MJD Start Times: 55263.26917722, 55263.40148125, 55263.53378520, 55263.66608919, 55263.73230484, 55263.79839317, 55263.79852053, 55263.86460888, 55263.93082453, 55263.99691291, 55264.12921690, 55264.26164819, 55264.39395223
No Detection
MJD Start Times: 55502.32299741, 55502.45530140, 55502.58747807, 55502.71978206, 55502.78587042, 55502.85208605, 55502.91817441, 55502.98426272, 55502.98439003, 55503.05047840, 55503.11656671, 55503.18278234, 55503.31495902, 55503.44726300, 55503.57956699
P/2008 R1 14 3.66 3.53 15.7 -‐-‐ -‐-‐
No Detection
MJD Start Times: 55518.82066059, 55518.95296481, 55519.08514168, 55519.08526905, 55519.21744591, 55519.28353439, 55519.28366170, 55519.34975019, 55519.41583862, 55519.48205441, 55519.54814285, 55519.68044707, 55519.81262398, 55519.94492822
MBOs, Bauer et al.
The WISE image data were processed using the scan/frame pipeline that applied instrumental, photometric, and astrometric calibrations (Cutri et al. × a Each object is listed per visit (see text). N refers to the number of exposures, R is the heliocentric distance of the AMBO in AU, Δ is the observer distance in AU, α is the phase angle in degrees. The “Coma?” column refers to whether there is apparent coma in the images. “Stacked?” indicates whether the analysis was performed on a stacked (co-‐added) image of the N exposures, to increase the signal from the AMBO, or whether each exposure was individually analyzed; No Detection indicates there was no detection in the stacked or individual images. MJD, i.e. the Modified Julian Date, is defined as JD-‐2400000.5. The range of dates for each listed visit is as follows (all dates are 2010): (596) Scheila (1 st visit) – Feb 15, 05:26:52 to Feb 16, 08:26:33; 133P – Mar 16, 23:59:57 to Mar 18, 02:59:38; P/2010 A2 – Apr 02, 06:37:08 to Apr 03, 19:08:11; 176P – Apr 23, 06:44:02 to Apr 24, 12:54:03; P/2010 R2 – Jun 09, 15:24:15 to Jun 11, 03:54:56; (596) Scheila (2 nd visit) – Nov 10, 21:15:04 to Nov 12, 03:24:21; 238P (1 st visit) – May 04, 12:39:00 to May 08, 23:01:27; P/2008 R1 (1 st visit) – Mar 08, 06:27:37 to Mar 09, 09:27:17; 238P (2 nd visit) – Nov 02, 07:45:07 to Nov 03, 13:54:35; P/2008 R1 (2 nd visit) – Nov 18, 19:41:45 to Nov 19, 22:40:42. MBOs, Bauer et al. binned W4 images; Wright et al. et al. For Scheila, 176P, 133P, and P/2010 R2, the image profiles including those of the stacked images were consistent with point-spread functions (PSFs), while the profile of P/2010 A2 was not consistent with a PSF, owing to the presence of coma. Special consideration was given to P/2010 R2 and to the December visit of Scheila (detected in the W1 and W2 bands only), owing to the fact that the WISE observations were relatively close in time to reported activity. No coma signal was apparent in the images (see Figure 1), nor was any significant coma signal found for P/2010 R2 or Scheila when surface brightness profile analysis techniques were applied (cf. Bauer et al.
MBOs, Bauer et al. were sampled out to 30 arcseconds, well into the region where each object’s SBP wings reach into the local background. Analysis also showed that fluxes for Scheila were consistent with the flux values derived from the February 2010 visit data when re-scaled for distance and phase angle (IAU phase parameter G=0.08; Bowell et al.
Table 2: Total Fluxes in mJy
Aperture photometry was performed on the stacked images of 176P, 133P, & P/2010 R2 for aperture radius values of 11 and 22 arcsec, the aperture sizes necessary to obtain the full signal from W3 and W4, the poorest resolution WISE bands, while pipeline-extracted magnitudes were used for the thermal fits of Scheila. The counts were converted to fluxes using the band-appropriate magnitude zero-points and 0 th magnitude flux values provided in Wright et al. (2010), and an iterative fitting to a black-body curve was conducted on the two long-wavelength bands to determine the appropriate color correction as listed in the same. The extracted magnitudes were then converted to fluxes (Wright et al. et al. et al. Object W1 (3.5 µ m) W2 (4.6 µ m) W3 (12 µ m) W4 (22 µ m) Log(Q CO2 /Q co) Scheila cryo post-cryo
MBOs, Bauer et al. profile-derived magnitudes providing there are no artifacts, saturation, or confusion with other sources in the images of the objects. Note that the profile magnitudes for Scheila allowed for a more accurate photometric magnitude in W3, since the core of the image was saturated for this object (cf. Mainzer et al. σ upper limits of fluxes in W1 and W2 for the remaining AMBOs with PSF-like profiles that lack detections in these bands. Corresponding CO and CO upper limits are also provided in units of log mol sec -1 , based on analysis outlined in Pittichova et al. (2008) and Bauer et al. (2011). The values listed assume a single source species for the observed upper limit in W2. Note that in the course of the analysis, the CO and CO values were re-computed for 103P/Hartley 2, and were found to be off by a factor of 17 as reported in Bauer et al. (2011) when using a higher precision code. The corrected column densities for CO and CO are 3.0 ( ± × and 3( ± × m -2 respectively. Production values therein should also be corrected as 6.0 ( ± × and 6.5 ( ± × mol sec -1 for CO and CO, respectively. The relative production rates for CO as compared to the predicted level of water production, then, are on the order of 20%, rather than the few percent stated in Bauer et al. (2011). The AMBO production upper limits compare with the103P CO production rate of 25.78 in log units. These AMBO production upper limits are weak constraints, owing to the greater distance of the AMBOs from WISE, relative to 103P, at each object’s times of observation. The AMBO flux upper limits are on the order of the expected confusion limits for each band (Cutri et al. σ upper limits of 2.0 km and 1.8 km (or 1- σ limits of 1.2 and 1.1 km) for 238P and P/2008 R1, respectively. We list the 1- σ upper limits in Table MBOs, Bauer et al.
3, and note that the limit is consistent with the 238P diameter estimate of 0.8 km by Hsieh et al. 2011b and that the region of sky containing the AMBO was observed by WISE during the reported time of inactivity. To constrain the albedo, H V values were obtained from the literature for 176P (Hsieh et al. et al. et al. et al. et al. et al. MBOs, Bauer et al. et al. (2011a) and found that the magnitudes matched those taken within a day of our 2010 Mar 15 observations. Jewitt et al. (2011a) also listed reported magnitudes within hours of the WISE April observations, with total magnitude H v v value as 15.1. However, project NEAT observe the predicted position of the AMBO on February 9, 2002 from the Palomar 48-inch, when the predicted R-band brightness was 20.1. The three images were provided by the NEAT archive project (cf. Lawrence et al. R =20.5, and no corresponding source was found near the predicted location. Therefore we instead used an H V for P/2010 R2 derived from observations taken at the University of Hawaii 2.2 meter telescope on Mauna Kea, HI on August 1 and 9, 2011 (UT) in place of the MPC’s listed value. These observations yielded an estimated magnitude of 23.8 +/- 0.1 in R-band (R=3.0 AU, Δ =3.5 AU, α =16 ° ), corresponding to the H v ~18.0, assuming an IAU phase slope parameter of G=0, and near-solar colors. This was the best available estimate; those at the MPC were based on observations obtained close to the time of apparent onset of activity. The measured colors of 133P (Hsieh et al. 2010), 176P (Hsieh et al. 2009), and (596) Scheila (pre-outburst; Tedesco 1995 and Yang & Hsieh 2011) are near-solar, within 0.06 magnitudes, and G-parameters are in the range of -0.1 to 0.1. Assuming an offset in both values (G=0.1 and V-R=0.42), the magnitude offset would be ~0.12, or approximately 12% in brightness, in MBOs, Bauer et al. both cases, considerably less than the uncertainty in albedo listed in Table 3, and on the order of the photometric uncertainty.
Discussion
Using a NEATM model (Harris et al. et al. et al. µ m signal alone. The sampling cadence of the MIPS data consisted of three exposures taken over an 8 minute interval for 133P and two exposures spaced 5 hours apart for 176P. As discussed previously, the WISE observations consisted of 13 and 16 visits for 133P and 176P respectively, spaced at 1.59 hour intervals and with a more complete sampling of each body's rotational phase. WISE observed both bodies far from the heliocentric distances about their perihelion where their activity was previously reported (Hsieh et al. 2010 & Hsieh et al. 2011a), and comparable to the distances of the Spitzer Space Telescope observations reported in Hsieh et al. 2009. The WISE data had two thermal channels at 12 and 22 µ m, which allowed for a fit with η as a free parameter, and the fits converged to solutions of η near 0.8. Considering these factors, the WISE results of size and albedo compare well with the Hsieh et al. 2009 results for the fixed η ~ 0.8. Converting the Hsieh et al. 2009 values of p R to p V , we derive for 133P an albedo of p V = 0.04 +/- 0.01, and for 176P, p V = 0.05 +/- 0.01, which overlap with our values in Table 3. Hsieh et al. (2009) report sizes ranging MBOs, Bauer et al. from 3.34 - 3.56 km for 133P and 3.44 - 4.08 km for 176P using η = 0.8 calculations, which overlap our derived values and uncertainties, although our sample likely falls closer to the mean, based on the WISE imaging cadence for each object. We used a default value of 2 for the ratio of p IR /p V in the thermal fit results. Note that the fits to this ratio are only loosely constrained by the W3 and W4 signal for 133P, 176P, and P/2010 R2, and so the value of p IR /p V is close to the default value used. For Scheila, however, p IR /p V was strongly constrained by the additional W1 and W2 signal, so that the ratio of 2 is a firmly fitted result, but not unlike what has been found for the WISE data for other redder (V-R > 0.36) main belt objects (cf. Mainzer et al. Table 3: Object Nucleus Thermal Fits
Object Diameter (km) p v p ir η Scheila 118 +/- 6 0.04 +/- 0.004 0.08 +/- 0.03 0.83 +/- 0.03 133P 3.2 +/- 0.2 0.06 +/- 0.02 0.12 +/- 0.03 0.8 +/- 0.1 176P 3.5 +/- 0.1 0.07 +/- 0.03 0.15 +/- 0.05 0.8 +/- 0.1 P/2010 R2 2.8 +/- 0.3 0.01 +/- 0.01 0.02 +/- 0.02 1.9 +/- 0.3 P/2010 R2 1.6 +/- 0.3 0.03 +/- 0.02 0.05 +/- 0.03
Fixed at ≤ ≥ Fixed at ≤ ≥ Fixed at
Physical parameters derived from the WISE data differ between the dust of active coma and solid nucleus surfaces. The general properties of the WISE data have been discussed in detail by Cutri et al. (2011), and the performance of thermal models applied to WISE observations of solid bodies is described in
Mainzer et al. (2011b,c). Methods used in the analysis of the coma dust particles of P/2010 A2 were similar to these introduced in Bauer et al. (2007, 2008, & 2011). Analysis of the flux of coma constrains the dust
MBOs, Bauer et al. particle size distribution and the quantity of CO and CO emitted by the comet. The IR fluxes for solid nuclei provide constraints on the size of the comet and, when accompanied by shorter (non-thermal) wavelength data, constrain surface albedo values as well. P/2010 A2 was the only AMBO in our sample to exhibit an apparent dust tail while WISE observed the body. As the coma dominated the signal, no special extraction of the nucleus signal was possible. Thermal fits to the coma of P/2010 A2 were conducted using a Planck function (Figure 4), similar to the analysis conducted on 103P/Hartley 2 (Bauer et al. et al. et al. µ m) grains that are highly absorbing in the optical but poorly emitting in the far-IR. Small grains are also likely to evince silicate emission bands (cf. Kolokolova et al. et al. et al . 2010, and Moreno et al . 2010. Since, as in Figure 1, the P/2010 A2 dust follows the P/2010 A2 orbit closely, larger (> 100 µ m) grains are likely dominant. While very near its perihelion (perihelion was on 2009 Dec 4 at a heliocentric MBOs, Bauer et al. distance of 2.0055 AU, outburst discovery heliocentric distance on 2009 Dec. 15 was 2.0061 AU), the activity in this AMBO is believed to have been initiated by an impact event (Jewitt et al. et al. production could be derived, but only upper limits. The near-simultaneous Hubble Space Telescope (HST) photometry (within hours of the WISE observations; Jewitt et al. et al. (2011), scaling by the effective projected area of the dust in the coma, we were able to find the number of dust particles (n dust ) contributing to the signal in the photometry aperture for each wavelength interval (Table 4). Assuming a new particle size in each band similar to the wavelength scale, and subtracting the contribution from each of the preceding band-centered wavelengths, starting with the longest (W4), we derived a particle mass distribution (PMD; d log n/d log m, with m as the particle mass with constant density ρ =1 g cm -3 ) as shown in Figure 5. A log-log slope was fit to the result, for a comparison with other comets, and found to be α = -0.5 +/- 0.1 (note that the aforementioned W3 excess does not significantly affect this fit), considerably more shallow than most active comets, which more commonly fall within the α = -0.8 to -1.2 range (Lisse et al. 1998, 2004, 2007; Fulle et al. et al. MBOs, Bauer et al. et al.
Table 4: Particle Mass Distribution
Quantity R-band 12 µ m 22 µ m m g [kg] 1.8 × -16 × -13 × -12 n g × × × Objects 133P, 176P, P/2010 R2, and Scheila all demonstrate image morphologies that matched the observational stellar PSFs. 176P and 133P were not close to their perihelia, i.e. when the bodies were observed to have been the most active in the past (cf. Hsieh et al 2010 & 2011). However, P/2010 R2 was approaching its perihelion distance of 2.62 AU and was observed to be active 65 days after WISE observed the object. It is still possible that P/2010 R2 was undergoing low-level activity at the time of the WISE observations in late June. The fact that the best fit beaming parameter for P/2010 R2 is 1.9 is suggestive, though other fits with lower η values are still feasible (See Figure 3D). Fits with η in the range of 0.8-1.2, although poorer, fall within the 95% confidence level and produce geometric albedo values in the range of 2-3.4%, closer to the 4-6% values of other AMBOs, rather than the lower 1% value for the best fit. We listed the P/2010 R2 fit for the fixed- η value of 0.8 in Table 3 in addition to the free- η fit for MBOs, Bauer et al. comparison with the other three AMBO fits, all of which yield η values near 0.8. For comparison, Fernandez et al. (2011) find the mean η value in their Jupiter family comet sample to be near 1.0, with a standard deviation of 0.1, significantly less than the high- η best fit for P/2010 R2. The best-fit high η value implies the temperature may be cooler than expected, and it may be cool enough to be explained by the presence of isothermal dust grains. Alternatively, the fit to a Planck function shown in Figure 3E is elevated by 27K from the black-body temperature, which may be caused by the same phenomena (abundant small or large grains or a pronounced silicate emission feature) discussed for P/2010 A2, or alternatively caused by a signal dominated more by the nucleus. If so, the nucleus size derived from the thermal fits would serve more as an upper limit, since activity would likely enhance the IR flux. The fact that the object shows no sign of activity in WISE data from surface brightness profile analysis could be at least partially due to the large pixel scales of the WISE data. The use here of an optical magnitude inferred from actual inactive data could have resulted in an underestimate the true optical magnitude at the time of the WISE observations, leading to an underestimated albedo. The limits of activity for Scheila are more firmly constrained in that the object size derived from the thermal flux, and the corresponding albedo, match those found in the literature (cf. Tedesco et al. MBOs, Bauer et al. ~3AU, and are equal with each other to within < 1% when corrected for heliocentric and observer distances listed in Table 1, and the observational phase angle change from 18.3 to 16.8 degrees, using an IAU slope parameter of G=0.08. Hence a clear and closer constraint is placed on the time of Scheila’s outburst, within 21 days of the earliest reported activity when the AMBO was ~1.3 magnitudes in excess brightness on 2010 Dec 3 (Jewitt et al. et al. et al. et al.
Conclusions
WISE has managed to sample the majority of the known AMBOs in the thermal and mid-IR. One AMBO, P/2010 A2, was dominated by its dust-coma signal, while the others were not likely active at the time of their observations. From the observed fluxes we conclude the following: • The thermal fits for P/2010 A2 yield higher temperature Planck functions than the black body temperature at the observed solar distance by 20% (38K), which can most readily be explained by large, non-isothermal grain dust. The slope of the
MBOs, Bauer et al.
PMD, in units of d log n/d log m is -0.5 ± 0.1. The PMD of P/2010 A2, when fit with a power law, is similar to that seen in an impulsive outburst, but most likely indicates that the activity was over a finite window of time several months in the past. • The onset time of activity for Scheila is further constrained by our data to be within 21 days of the first observation of activity. The derived surface reflectance and diameter are consistent with literature values (p V =0.04+/-0.008, D=115+/-6, cf. Tedesco et al. • AMBO nuclei albedos are consistent with measured comet albedos, i.e. are on the order of a few percent.
Acknowledgements
This publication makes use of data products from the Wide-field Infrared Survey Explore, which is a joint project of the University of California, Los Angeles, and the Jet Propulsion Laboratory/California Institute of Technology, funded by the National Aeronautics and Space Administration. This publication also makes use of data products from NEOWISE, which is a project of the Jet Propulsion Laboratory/California Institute of Technology, funded by the Planetary Science Division of the National Aeronautics and Space Administration. NEAT archive data was provided through NASA’s Planetary
MBOs, Bauer et al.
Mission Data Analysis Program. Observing time was allocated at Steward Observatory’s 0.9m (Spacewatch) telescope on Kitt Peak . J. Bauer would also like to thank Drs. Hsieh and Jewitt for their valuable discussions regarding AMBOs. This material is based in part upon work supported by the NASA through the NASA Astrobiology Institute under Cooperative Agreement No. NNA09DA77A issued through the Office of Space Science.
References
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Figure Captions
Figure 1
A three-color composite image of
AMBOs from the WISE data. The stacked 4.6, 11.6, and 22.1 µ m images were mapped to blue, red, and green channels The AMBOs from left to right are (top row) P/2010 A2 and P/2010 R2, and (bottom row) Scheila, 176P, and 133P. The P/2010 A2 image is 9 arcmin across its bottom edge, while the others are 4.5 arcmins across. The P/2010 A2 panel shows the sky-projected anti-solar vector as indicated by the green dashed arrow, and the projected anti-velocity vector by the white dotted arrow. Note that while Scheila is saturated in W3, the profile photometry used in our analysis is still viable (Cutri et al. Figure 2
Surface brightness profiles for Scheila in W2 (top panel) and P/2010 R2 in W3 (bottom panel) sampled out to 24 arcseconds. The PSFs were constructed from a nearby bright star in the images for P/2010 R2. For Scheila, which had no nearby stellar counterpart of similar brightness, the comparison PSF was constructed from the array of synthetic PSFs available from the WSDS (Cutri et al.
MBOs, Bauer et al. the pixel location of Scheila on each image. Error bars shown include the photometric uncertainties of the objects as well as the uncertainty in the background level. Note that owing to the considerably lower signal-to-noise-ratio for P/2010 R2, the SPB is more coarsely sampled, i.e. binned over twice the interval than that used for Scheila. Magnitude values are instrumental, based on image counts and default zero points (cf. Wright et al.
Figure 3
The thermal fits for the AMBOs exhibiting PSF-shaped profiles: 133P (panel A), 176P (panel B), Scheila (Panel C) and P/2010 R2 (Panels D & E). Panels A-D show fits using the NEATM model appropriate for signal dominated by solid nuclei (Harris et al. et al. η =0.8 (dotted lines), η = 1.0 (dashed lines), and η =1.2 (dot-dahsed lines). Note that in Panel C, the fit to 596 converged freely to η =0.8 (see Tabel 3 for the best-fit parameters), and that for panel D we include the best-fit model of η =1.9 (dot-long-danshed line); the η values were otherwise fixed for the fits shown. The drived diameters (D [km units]) and albedos (p v ) are also shown in the lower right of each of these panels for each model fit. Panel E shows the black-body fit to P/2010 R2, appropriate for a dust-coma dominated signal, through no apparent extended profile was found in the stacked image (see text). Figure 4
Coma temperature fit to the 4.3 arcmin aperture thermal photometry in the two longest WISE wavelength bands of the P/2010 A2 observations. A reflected-light model with a neutral reflectance (heavy dotted) based on the near-simultaneous photometry from Jewitt et al. (2011a) is shown along with the combined signal (dashed line). The uncertainties to the temperature fit are on the order of +/- 9K, and the fitted temperature (238K) is in excess of the black body temperature for that distance (200K).
Figure 5
Particle Mass Distribution (PMD) of P/2010 A2. Log number is shown vertically, while log mass is shown on the bottom scale and the corresponding grain radius size, in microns, is shown on the scale above. The P/2010 A2 data derived number of particles in the 4.3 arcmin aperture radius (stars), encompassing the complete signal from the dust tail, are shown. For comparison, 103P/Hartley 2 (pentagons; Bauer et al. et al. et al. et al. et al. α = -0.75, in log N/log kg units, where N is the estimated total number of dust grains in the aperture; Green et al. ρ size. Echeclus’ PMD best-fit ( α = -0.87) is shown as a dotted line, and the solid line is the best fit to 103P PMD data ( α =-0.97). The dot-dashed line represents the best fit to the P/2010 A2 data of α =-0.5. MBOs, Bauer et al.
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Figures
Figure 1
N E ν P/2010 A2 P/2010 R2 176P (596) Scheila 133P
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Figure 2 (596) Scheila P/2010 R2
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Figure 3
A B C D E
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Figure 4
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