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What ’ s the big deal about Comet ISON?

X. What ’ s the big deal about Comet ISON?. Matthew Knight Lowell Observatory* knight@lowell.edu. Probably won ’ t be the comet of the century, but it should be very interesting anyway. *And long-term visitor at JHU-APL. Outline. Quick background on comets

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What ’ s the big deal about Comet ISON?

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  1. X What’s the big deal about Comet ISON? Matthew Knight Lowell Observatory* knight@lowell.edu Probably won’t be the comet of the century, but it should be very interesting anyway *And long-term visitor at JHU-APL

  2. Outline • Quick background on comets • Previous studies of sungrazing comets • What do we know about ISON so far? • When will [your favorite observing platform] be able to see ISON?

  3. Comet Basics Gas tail Nucleus Dust tail Coma Comet Hale-Bopp from Google Image Search

  4. Why study comets? • Comets are remnants of the formation of the solar system • Stored in the outer solar system • Largely unchanged since formation • Contain ice, dust, and organics • Building blocks of the planets • Ingredients for life • Hazardous to life on Earth Images from Google image search

  5. Why ISON? • Likely its first passage through the inner solar system • Discovered at a large enough distance to study in more detail than most “dynamically new” comets • On a “sungrazing” orbit that will bring it very close to the Sun • Discovered much earlier than any previous sungrazer • Perihelion Nov 28, 2013 at a distance of 2.7 solar radii • Projected to get very bright near perihelion • Favorable viewing geometry for observing from Earth post-perihelion Dec 25, 2013 Image credit: NASA GSFC Scientific Visualization Studio

  6. Why are sungrazers special? • Different temperature and stress regime than typical comets • Reveals least volatile components of solar system • Fragmentation is common • High phase angles that can’t be seen elsewhere in the solar system • Yields unique information about dust properties • “Solar probes” that can inform studies of the solar wind, magnetic field, corona, etc. C/2011 W3 Lovejoy Image credit: helioviewer.org

  7. Sungrazing comet basics • Perihelion distance less than a few solar radii • 1 solar radius = 0.0046524 AU ≈ 700,000 km • A handful of historically bright comets in history • Most are dynamically related to each other as members of the Kreutz group • Coronagraphic observations over last ~30 years have revealed a steady stream of small fragments • Most do not survive perihelion • Typical observations span hours to days • Includes several new groups of “sunskirting” comets Top: Ikeya-Seki in 1965, Bottom: Great Comet of 1680 from Google Image Search

  8. White light studies Knight et al., 2010, AJ, 129, 936 • SMM, Solwind, SOHO (LASCO), STEREO (SECCHI) • Results • Vast majority of all sungrazing comet discoveries • Orbital determination • Measurement of brightness to infer sizes • Basic compositional information via color filters • Tail dynamics • e.g., Biesecker et al. 2002 (Icarus, 157, 323), Knight et al. 2010 (AJ, 129, 936), Thompson 2009 (Icarus, 200, 351), many papers by Sekanina Thompson, 2009, Icarus, 200, 351

  9. UV • SOHO/UVCS • Spectroscopic studies of a few comets that survive to <8 Rsun • e.g., Raymond et al. 1998 (ApJ, 508, 410), Bemporad et al. 2007 (P&SS, 55, 1021) • SOHO/SWAN • (Almost) all sky H- imager used to infer water production rate in comets • Rarely sees sungrazers • e.g., Combi et al. 2011 (Icarus, 216, 449) Kohl et al., 2001, Solar Physics, 200, CD-ROM C/2012 E2 in SOHO/SWAN

  10. EUV • SDO/AIA and STEREO/EUVI • Observation of two sungrazers that survived until ~100,000 km above photosphere (Schrijver et al. 2011, Science, 335, 324; Bryans & Pesnell 2012, ApJ, 760, 18; McCauley et al. 2013, ApJ, 768, 161) • “Solar probes” that inform studies of the solar wind, magnetic field, corona, etc. Schrijver et al. 2011, Science, 335, 324

  11. Sungrazer observations • Vast majority of all sungrazing comet observations have been serendipitous • Solar facilities not optimized for comet observations • SOHO, STEREO, SDO, etc. • Only sungrazer of the modern era that was known in advance was Comet Lovejoy (C/2011 W3) • Lovejoy only discovered ~3 weeks before perihelion, disintegrated shortly after perihelion • Largest sungrazing comet since 1970 (until ISON) • ISON is the first sungrazing comet ever discovered beyond 1 AU • Unique opportunity to methodically study a sungrazer

  12. ISON’s Discovery • Discovered September 21, 2012 by Vitali Nevski and Artyom Novichonok • Used 0.4-m telescope at International Scientific Optical Network (ISON) near Kislovodsk, Russia • Pre-covery image from Mt Lemmon on Dec 28, 2011 Image credit: Google Maps/image search

  13. Early observations very important • Beyond the water line at ~3 AU so current activity driven by something less volatile • Probably CO or CO2, but not well studied • Will soon be unobservable with traditional telescopes • Activity likely to be too high when next observable (September) to detect the nucleus • Nucleus size critical for predicting if it will survive perihelion

  14. Observations so far • Most observations just broadband brightness • Deep Impact Flyby spacecraft successfully observed it but could not detect CO or CO2 (too faint) • Only measured gas production rates of which I’m aware: • QH2O<51025 mol/s on Jan 30 (Bodewits with SWIFT) • QH2O~1-101026 mol/s on Mar 5 (Schleicher 2013) • Production rates suggest nucleus could be as small as a few hundred meters (but probably factor of a few larger) Image Credit: JPL Horizons

  15. Hubble observations April 10 • Exceptional spatial resolution of HST allows best chance of separating nucleus from coma • Nucleus <2 km in radius • No variations due to nucleus rotation detected • Enhancement reveals a sunward jet Image credit: Li, Weaver, Knight, et al.

  16. Will It Survive? • Two considerations: erosion and disruption • Kreutz comets generally do not survive in tact • Small comets seen by SOHO erode away completely before perihelion • Medium sized historical ones often disappear soon after perihelion and appear “headless” • Lovejoy survived for a few hours to days before disrupting catastrophically (Sekanina & Chodas 2012, ApJ, 757, 127; Knight et al., in prep) • Historically bright ones survive but fragment • “String of pearls” seen in Great Comet of 1882 • Multiple fragments seen in Ikeya-Seki, probably Great Comet of 1843 and Pereyra (1963)

  17. Erosion • Most of energy goes into evaporation • Latent heat of sublimation for silicates, organics are relatively similar to water ice (e.g. Sekanina 2003, ApJ, 597, 1237; Brown et al. 2011, A&A, 535, 71) • Will likely erode 100-200 m of radius From D. Pesnell • Well beyond region where ablation matters (1.01 Rsun; Brown et al. 2011) • Rate of erosion is comparable to rate of heat wave propagating into interior so effectively always exposing “new” ice (Knight et al. 2010, ApJ, 139, 926; Gundlach et al. 2012, arXiv:1203.1808) • i.e., shouldn’t have volatile depletion due to evolution

  18. Disruption Roche Limit • Distance at which a secondary body will be disrupted due to tidal forces from a primary object • This is what caused Shoemaker-Levy 9 to disrupt around Jupiter in 1992 Weaver & Smith via APOD Image credit: Wikipedia

  19. Disruption • Roche limit calculation: • sun = 1.4 g/cm3,comet = 0.4 g/cm3  d = 3.7 Rsun • Simple analysis says ISON will be within Roche Limit • But, Sridhar & Tremaine (1992, Icarus, 95, 86) showed that a strengthless body begins to shed mass at 0.69*Roche Limit (also: Asphaug & Benz 1996, Icarus, 121, 225) • 0.69*3.7 Rsun = 2.6 Rsun • Implies ISON will be right on the edge of shedding mass • If ISON’s density is higher than 0.4 g/cm3 then it is more likely to survive • Gundlach et al. (2012, arXiv:1203.1808) argued that reaction force from sublimation of ice can prevent tidal splitting

  20. So…will it survive? • Big enough to survive erosion if not near small end of size range (radius ~0.2-2.0 km) • Very close to the Roche limit so may disrupt • This may tell us about its internal strength • If it disrupts, erosion will work much more efficiently on individual fragments

  21. What can we expect to see? • Potential to be visible during daytime/twilight • Visible from northern hemisphere • Lovejoy is a good analog although it was probably ~2-5x smaller

  22. Lovejoy in SOHO images

  23. Lovejoy 1 hour before perihelion (r=0.013 AU) Nucleus

  24. Lovejoy 2 hr after perihelion (r=0.021 AU) • Loss of tail due to short lifetimes of grains (c.f. Huebner 1970, A&A, 5, 286) • Also observed in Ikeya-Seki Nucleus

  25. Lovejoy 2.5 days after perihelion Venus Jupiter STEREO HI1-A

  26. Lovejoy 2-3 weeks later Images from Google image search

  27. Lovejoy 3 weeks after perihelion (HST) Image from Siding Spring Observatory (courtesy R. McNaught) JPL Horizons predicted nucleus position Hubble field of view No more nucleus! Image from HST WFC3 (Knight, Weaver, et al.)

  28. Within minutes of perihelion McCauley et al. 2013, ApJ, 768, 161 SDO-AIA (EUV) Hinode (optical) http://sungrazer.nrl.navy.mil/index.php?p=news/birthday_comet

  29. Predicted path HI1A outbound Dec 1-7 HI2A inbound Oct 10-Nov 23 HI1A inbound Nov 21-28 Images on this and subsequent slide from http://stereo-ssc.nascom.nasa.gov/comet_ison/

  30. STEREO COR2B Nov 26-30 STEREO COR2A Nov 28 SOHO C2 & C3 Nov 27-30 STEREO EUVI-B Nov 28

  31. Will SDO/AIA see it? • SDO will be pointed so that ISON passes through field of view • SDO sees comets as elements (mostly oxygen) are rapidly ionized in T~106 K environment • Primarily through collisions with free electrons in corona • ISON only reaches ~2.7 Rsun • Previous two comets observed by SDO/AIA reached 1.1-1.2 Rsun • Will ISON’s larger size compensate for the significantly lower density of the corona? • Lower temperature also helps because lifetimes of ions are longer ? NASA/SDO

  32. Observability • Excellent observing circumstances post-perihelion for northern hemisphere • Morning object near perihelion • Up all night by late-December C/2006 P1 McNaught (in 2007) C/2011 W3 Lovejoy (in 2011) Images from Google image search

  33. Visible • Typical “night time” telescopes have individual constraints, but probably >30º elongation • Observable Jan-Jun 2013, Aug-Nov 2013, Dec 2013 onward • Key science: composition, coma morphology, secular lightcurve Comet Lovejoy at VLT by Gabe Brammer

  34. Solar Telescopes • Will be able to observe essentially continuously near perihelion • e.g. McMath-Pierce at KPNO, Sacramento Peak Observatory, Big Bear Observatory • Key science: spectroscopy of metals • Many metals become active <0.15 AU • Only comet ever observed like this was Ikeya-Seki in 1965 Preston (1967, ApJ, 147, 718) spectra of Ikeya-Seki

  35. IR • Ability to observe during the day, e.g. IRTF which can go ~18° from Sun • Allows observations through approximately ±2 days around perihelion • Key science: dust, composition Image credit: Taylor Chonis

  36. Radio/mm • Minimal pointing restrictions so can observe essentially all the way through perihelion • Key science: production rates all the way through perihelion Biver et al., 2002, EM&P, 90, 5

  37. When will it be observable (approximate)? Solar telescopes IR radio Optical

  38. Space telescopes • Hubble • Already observed in April, additional observations planned in May, October, post-perihelion • Key science: nucleus size estimate, composition, fragment search • Spitzer • Solar elongation constraints: May-June 2013, Jan-Feb 2014 • Key science: dust properties, constrain nucleus size • Swift • Already observed in Jan/Feb, monitoring throughout apparition • Key science: production rates • Herschel • Already observed in March • Key science: composition, nucleus size • X-ray telescopes (Chandra, XMM, Suzaku, etc.) • Minimal pointing constraints • Key science: Solar wind diagnostics, possible comet/corona interaction

  39. Other NASA assets • BRRISON balloon experiment (October) • 120,000 ft altitude allows some UV/IR not possible from ground • Deep Impact Flyby spacecraft (ongoing) • Unique ability to measure CO/CO2/H2O • Continuous monitoring over many days possible - potentially yields lightcurve and rotation period • Observes ISON when not possible from ground • Various Mars missions (October ~2) • Close approach (0.07 AU) so high spatial resolution • MESSENGER at Mercury (November ~19) • More favorable viewing geometry than Earth at the same time

  40. Will ISON live up to expectations? • Projections now (solid line) somewhat less optimistic than in late 2012 (dotted line) • Activity still consistent with it reaching mag<0 • Nucleus size and internal strength will dictate if/how well it survives perihelion Naked eye

  41. Will ISON live up to expectations? • There is always a chance it will underperform (e.g. Kohoutek in 1973) or disintegrate completely (e.g. Elenin in 2011) • Even if it massively underperforms it should still be very impressive in SOHO/STEREO images • Should have a better idea of the behavior by August when water will have turned on Kohoutek 1/12/74 at Palomar

  42. Conclusions • Due to its sungrazing orbit, ISON has the potential to be very bright • Excellent viewing geometry in December-January • Early observations suggest nucleus is <2 km • Uncertain if/how well it will survive perihelion • ISON is a unique opportunity to study a sungrazing comet in great detail • First opportunity of the modern telescopic era to observe a sungrazing comet at many wavelengths • Can study the least volatile components of the solar system • Can look for a change in the composition due to the intense heating at perihelion • If it fragments, may be able to look for heterogeneity

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