EUV OBSERVATIONS OF SUNGRAZING COMETS WITH THE SOHO/UVCS INSTRUMENT

1. Arcetri Astrophysical Observatory Harvard-Smithsonian CFA Torino Astrophysical Observatory University of Florence “EUV observations of sungrazing comets with the SOHO/UVCS instrument” EUV OBSERVATIONS OF SUNGRAZING COMETS WITH THE SOHO/UVCS INSTRUMENT A. Bemporad1, G. Poletto2, J. C. Raymond3 and S. Giordano4 1Astronomy and Space Science Dept. – University of Firenze – Firenze, Italy 2INAF - Arcetri Astrophysical Observatory – Firenze, Italy 3Harvard - Smithsonian Center for Astrophysics, Cambridge, MA, USA 4INAF – Torino Astrophysical Observatory, Torino, Italy AOGS 2nd Annual Meeting 2005 June 20-24, 2005 — Suntec, Singapore

2. “EUV observations of sungrazing comets with the SOHO/UVCS instrument” SUMMARY • Introduction: state of art • The origin of the observed cometary emission • Determination of cometary parameters • Results: sungrazers outgassing rate, nuclear size, • fragmentation processes, bow shock • Work in progress AOGS 2nd Annual Meeting 2005 June 20-24, 2005 — Suntec, Singapore

3. “EUV observations of sungrazing comets with the SOHO/UVCS instrument” INTRODUCTION: BEFORE & AFTER THE SOHO MISSION (1995) • Before 1979 only about 12 sungrazers were discovered: 3 before the IX century, 5 in the IX century and 4 in the years 1945-1970. • In the decade 1979-1989 about 16 sungrazers were discovered: 6 by the SOLWIND instrument on the P78-1 satellite and 10 by the coronagraph aboard the Solar Maximum Mission. • Since 1996 the two SOHO/LASCO coronagraphs discovered more than 900 comets; over 700 belong to the Kreutz sungrazer group. AOGS 2nd Annual Meeting 2005 June 20-24, 2005 — Suntec, Singapore

4. “EUV observations of sungrazing comets with the SOHO/UVCS instrument” THE LASCO (Large Angle Spectroscopic Coronagraph) INSTRUMENT ON SOHO C/1996 Y1 C/1996 Y1 LASCO/C3: from 3.7 to 32 Rsun CCD 10242 - Pixel size: 56.0” LASCO/C2: from 2.0 to 6.0 Rsun CCD 10242 - Pixel size: 11.4” AOGS 2nd Annual Meeting 2005 June 20-24, 2005 — Suntec, Singapore

5. “EUV observations of sungrazing comets with the SOHO/UVCS instrument” THE UVCS (UltraViolet Coronagraph Spectrometer) INSTRUMENT ON SOHO UVCS: from 1.4 to 10 Rsun - CCD 1024X360 Pixel size: 0.0925Å spectral, 7” spatial AOGS 2nd Annual Meeting 2005 June 20-24, 2005 — Suntec, Singapore

6. Year Comets # Obs. Date Comet Name Comet Group Published 1996 1 Dec 23 C/1996 Y1 SOHO-6 KreutzYes 1997 1 May 1-2 C/1997 H2 SOHO-8 None No 1998 1 Jun 1 C/1998 K10 SOHO-54 Kreutz No 1999 2 May 20 C/1999 K1 SOHO-63 Kreutz No Sep 17 C/1999 S1 SOHO-86 Kreutz No 2000 4 Feb 10 C/2000 C6 SOHO-104 KreutzYes Feb 29 C/2000 D1 SOHO-106 Kreutz No Sep 9-11 2P/ENCKE None Yes Oct 11 C/2000 T1 SOHO-204 Kreutz No 2001 1 Feb 6-7 C/2001 C2 SOHO-294 KreutzYes 2002 2 May 14 C/2002 J8 SOHO-442 Kreutz No Sep 18 C/2002 S2 SOHO-517 Kreutz No 2003 1 Jan 27-29 C/2002 X5 Kudo-Fujikawa None Yes “EUV observations of sungrazing comets with the SOHO/UVCS instrument” UVCS SUNGRAZER OBSERVATIONS: STATE OF ART Sungrazing comets are important to understand physical evolution and disintegration of comets, but… • UVCS observed only 13 comets: 10 of these belong to the Kreutz Group. • Until now, only 3 works have been published. AOGS 2nd Annual Meeting 2005 June 20-24, 2005 — Suntec, Singapore

7. “EUV observations of sungrazing comets with the SOHO/UVCS instrument” UVCS SUNGRAZER OBSERVATIONS: DISCUSSION PLAN • UVCS observations of sungrazers revealed a high cometary emission in the Hydrogen Lyman-α λ1216Å spectral line. This implies an enhanced number of H atoms. • Cometary H atoms may be created by different processes: from an analysis of the Lyα profile we can infer their origin. • From the observed Lyα emission we derive an estimate for the cometary outgassing rate and the nucleus size. • From the variation with the heliocentric distance of the outgassing rate we discuss possible fragmentation processes. AOGS 2nd Annual Meeting 2005 June 20-24, 2005 — Suntec, Singapore

8. Cometary Lyα emission only from Radiative excitation “EUV observations of sungrazing comets with the SOHO/UVCS instrument” THE ORIGIN OF THE Lyα EMISSION IN SUNGRAZING COMETS • In coronal conditions spectral lines emission is mainly due to: • Collisional excitation with thermal electrons • Radiative excitation from the chromospheric radiation • In order to distinguish between these processes, we can look at the ratio • Lyβ/Lyα between the H Lyman-β λ1025Å, also included in the UVCS spectra, and the Lyman-α spectral lines. At typical coronal temperatures (~106 K): collisional Lyβ/Lyα ~ 0.13-0.14 Because of the small chromospheric ratio: radiative Lyβ/Lyα ~ 0.001-0.002 In the UVCS observations of C/1996 Y1, C/2000 C6 and C/2001 C2 the Lyβ line was absent AOGS 2nd Annual Meeting 2005 June 20-24, 2005 — Suntec, Singapore

9. H atoms from H2O photodissociationmove with the comet Secondary H atoms have the coronal plasma velocity distribution “EUV observations of sungrazing comets with the SOHO/UVCS instrument” THE ORIGIN OF THE Lyα EMISSION IN SUNGRAZING COMETS • H atoms responsible for this Lyα emission may arise from: • Photodissociation of outgassed H2O molecules • Charge exchange process between coronal p+ and H atoms from H2O • The velocity imparted to the H atoms from outgassing (Delsemme 1982) and photodissociation processes (Huebner 1992) is ~ 30-40 km/s, much smaller than typical sungrazer speed close to the Sun. • The momentum transfer in the charge exchange process between p+ and H atoms is very small (McClure 1966). AOGS 2nd Annual Meeting 2005 June 20-24, 2005 — Suntec, Singapore

10. “EUV observations of sungrazing comets with the SOHO/UVCS instrument” THE ORIGIN OF THE Lyα EMISSION IN SUNGRAZING COMETS Hence Lyα emission from H atoms generated by H2O photodissociation: • Is strongly reduced by the Swing effect (dimmed by a factor 0.2-0.1 for • vrad ~ 250-300 km/s, Kohl 1997) • Would drop as soon as the comet travels beyond the spectrograph slit, • while UVCS observations revealed a persistent (30-40 minutes) Lyα emission. • Would results in a narrow line profile, while… AOGS 2nd Annual Meeting 2005 June 20-24, 2005 — Suntec, Singapore (from Uzzo et al. 2001)

11. H atoms responsible for the sungrazer emission form from the charge exhange between coronal p+ and H atoms from H2O “EUV observations of sungrazing comets with the SOHO/UVCS instrument” THE ORIGIN OF THE Lyα EMISSION IN SUNGRAZING COMETS …while the FWHM of the cometary and coronal Lyα profiles are about equal. (from Uzzo et al. 2001) AOGS 2nd Annual Meeting 2005 June 20-24, 2005 — Suntec, Singapore

12. From the observed (Lyα)countsvs. time curve we derive the cometary outgassing rate and the local plasma electron density ne “EUV observations of sungrazing comets with the SOHO/UVCS instrument” ESTIMATE OF SUNGRAZERS OUTGASSING RATE AND NUCLEUS SIZE • The comet leaves along its path a number of neutral H atoms Ncoma • proportional to the outgassing rate Ndot and to the charge exchange rate • cx (which depends on the proton flux and the local proton density np= ne ). • Then, after the comet transit, the total number of Lyα counts • exponentially decays with time as exp(-t/ion) , where ion is the ionization • rate (mainly by collisions with electrons, ion∝ ne-1). AOGS 2nd Annual Meeting 2005 June 20-24, 2005 — Suntec, Singapore (from Bemporad et al. 2005)

13. From the observed (Lyα)countsvs. time curve we derive the cometary outgassing rate and the local plasma electron density ne Assuming a spherical nucleus and isotropic outgassing we estimate its equivalentradius “EUV observations of sungrazing comets with the SOHO/UVCS instrument” ESTIMATE OF SUNGRAZERS OUTGASSING RATE AND NUCLEUS SIZE • The comet leaves along its path a number of neutral H atoms Ncoma • proportional to the outgassing rate Ndot and to the charge exchange rate • cx (which depends on the proton flux and the local proton density np= ne ). • Then, after the comet transit, the total number of Lyα counts • exponentially decays with time as exp(-t/ion) , where ion is the ionization • rate (mainly by collisions with electrons, ion∝ ne-1). AOGS 2nd Annual Meeting 2005 June 20-24, 2005 — Suntec, Singapore

14. Comet name QH2O (kg/s) r (m) h (Ro) C/1996 Y1 20.0 3.4 6.80 C/2000 C6 71.8 3.0 3.26 140 5.8 4.56 34.6 3.4 5.71 10.5 2.5 6.36 C/2001 C2 820a 20.3 3.60 58.9a 7.8 4.98 28.5b 5.4 4.98 UVCS observations revealed a “hidden mass” inconsistent with nominal models of coronagraphic observations and explained by slowly eroding subfragments (Sekanina 2003). The observed sudden increases in the Lyα brightness could indicate the fragmentation of the comet nucleus (Uzzo et al. 2001). “EUV observations of sungrazing comets with the SOHO/UVCS instrument” UVCS OBSERVATION OF SUNGRAZERS: RESULTS (LASCO running difference movies) AOGS 2nd Annual Meeting 2005 June 20-24, 2005 — Suntec, Singapore

15. 1. Occasional secondary brightenings in some sungrazer lightcurves have been explained by introducing one or more nearby companions traveling with the main nucleus (Sekanina 2003) 2. From a comparison between the orbital parameters of different comets, has been demonstrated (Sekanina 2002) that many sungrazers arriving in pairs or triplets originated via fragmentation events from a single sungrazer far from the Sun. (from Sekanina 2003) “EUV observations of sungrazing comets with the SOHO/UVCS instrument” SUNGRAZING COMETS: FRAGMENTATION PROCESSES Fragmentation processes play a key role in the origin and evolution of sungrazing comets. AOGS 2nd Annual Meeting 2005 June 20-24, 2005 — Suntec, Singapore

16. Close to the Sun, sungrazer fragmentation is more probable because of: a) High tidal stresses (∝r2/d3 ), in particular around the Roche limit (~ 3.2 Ro) “EUV observations of sungrazing comets with the SOHO/UVCS instrument” SUNGRAZING COMETS: FRAGMENTATION PROCESSES 3. Recently an intriguing scenario has been published (Sekanina & Chodas 2004) involving splitting both close and far to the Sun. This could explain the generation, via runway frag-mentation, of the observed sungra-zers from a single progenitor. (from Sekanina & Chodas 2004) b) High thermal stresses, mainly on the nucleus surface c) High sublimation flux can create an average surface pressure overcoming the tensile strength AOGS 2nd Annual Meeting 2005 June 20-24, 2005 — Suntec, Singapore

17. Tail 1 Tail 2 2) Dust particles ejected from a single object at 2 different times: we cannot exclude this interpretation, even if in this scenario it’s not easy to explain why ~1h later the comet shows only one tail at 3.60 Ro. “EUV observations of sungrazing comets with the SOHO/UVCS instrument” SUNGRAZING COMETS: THE OBSERVED FRAGMENTATION OF C/2001 C2 UVCS observations of C/2001 C2 at 4.98 Ro show the presence of 2 Lyα tails. A first interpretation is that the comet is composed by two fragments, but at least 2 more alternatives are possible: 1) 2 tails generated by a single object: however, at 5 Ro the nucleus is exposed to an extremely high solar flux (~ 2.5·109ergscm-2 s-1) → the whole surface should be active. (from Bemporad et al. 2005) AOGS 2nd Annual Meeting 2005 June 20-24, 2005 — Suntec, Singapore

18. This interpretation seems to be confirmed by the C/2001 C2 lightcurve which shows a secondary brightnening below ~ 5.3 Ro. “EUV observations of sungrazing comets with the SOHO/UVCS instrument” SUNGRAZING COMETS: THE OBSERVED FRAGMENTATION OF C/2001 C2 We interpret the two observed Lyα tails as two fragments, unresolved by the LASCO images. These fragments had an equivalent radius of 7.8 m and 5.4 m and were ~ 90200 km apart( ~ 124”). AOGS 2nd Annual Meeting 2005 June 20-24, 2005 — Suntec, Singapore

19. (from http://www.astro.uwo.ca/~jlandstr/planets/) Sungrazer C/1996 Y1 crossed a fast wind region: its Lyα line profile was much broader (Tk~ 9 ·106 K) than the background coronal profile. From the observed proton temperature, Raymond et al. 1998 inferred Vwind ~ 620 km/s “EUV observations of sungrazing comets with the SOHO/UVCS instrument” SUNGRAZING COMETS: BOW SHOCK Close to the Sun, magnetic field, solar wind velocity Vwind and the sungrazer velocity Vcomet are nearly radial. A shock should form with a shock velocity Vshock= Vwind + Vcomet . A fraction of the energy dissipated in the shock goes into plasma heating. Sungrazers C/2000 C6 and C/2001 C2 were immersed in a slow wind region; no significant plasma heating was observed from Lyα profiles. AOGS 2nd Annual Meeting 2005 June 20-24, 2005 — Suntec, Singapore

20. UVCS observations of sungrazer C/2002 S2 revealed the presence of 2 Lyα tails at 6.84 Ro and possibly also at lower heights. (S. Giordano, private comunication) “EUV observations of sungrazing comets with the SOHO/UVCS instrument” C/2002 S2 SUNGRAZING COMET: WORK IN PROGRESS On September 18-19, 2002 UVCS observed the sun-grazer C/2002 S2 at 4 heliocentric distances. LASCO/C2 & C3 movie AOGS 2nd Annual Meeting 2005 June 20-24, 2005 — Suntec, Singapore

21. The puzzling Lyα Doppler shift image revealed the 2 observed tails to be red- and blue-shifted by ~ 100 km/s. These cannot be interpreted as the signature of 2 fragments. “EUV observations of sungrazing comets with the SOHO/UVCS instrument” C/2002 S2 SUNGRAZING COMET: OPEN QUESTIONS The observed curves for the Lyα counts vs. time show a slow increase, difficult to explain with the actual model. AOGS 2nd Annual Meeting 2005 June 20-24, 2005 — Suntec, Singapore

22. vwind = 0 km/s ne = 1.4 · 104 cm-3 Tk = 1.5 · 106 K Ndot = 1 · 1028 s-1 vwind = 200 km/s ne = 2.8 · 104 cm-3 Tk = 1.5 · 106 K Ndot = 1 · 1028 s-1 “EUV observations of sungrazing comets with the SOHO/UVCS instrument” C/2002 S2 SUNGRAZING COMET: THE MODEL Actually the formulation of a simple model to explain the observed Lyα features is in progress. AOGS 2nd Annual Meeting 2005 June 20-24, 2005 — Suntec, Singapore

23. “EUV observations of sungrazing comets with the SOHO/UVCS instrument” CONCLUSIONS (1/2) • Since 1995 LASCO discovered more than 700 sungrazers; however only 10 of these were observed also by UVCS and only 3 works are published. • UVCS observations of sungrazers revealed a high cometary emission in the Hydrogen Lyman-α λ1216Å spectral line. H atoms responsible for this emission are those formed with a charge exchange process between H atoms from H2O and coronal protons. • From UVCS data it’s possible to give an estimate for the cometary outgassing rate and the equivalent radius: the observed sungrazers have a radius of about 3 – 8 m below ~ 6 Rsun (“hidden mass”). AOGS 2nd Annual Meeting 2005 June 20-24, 2005 — Suntec, Singapore

24. “EUV observations of sungrazing comets with the SOHO/UVCS instrument” CONCLUSIONS (2/2) • The observed increases in the outgassing rate indicate fragmentation. • Taking advantage of the UVCS higher spatial resolution (e.g. 42” for C/2001 C2), the presence of sub-fragments, unresolved by LASCO coronagraphs, has been directly observed for the C/2001 C2 sungrazer. • The formation of the cometary bow shock is strongly dependent on the coronal region encountered by the comet. • The explanation for the puzzling behaviour of the Lyman-α emission from the C/2002 S2 sungrazer is actually a “work in progress”: a good modelof the cometary tail could help us in order to explain the observed features. AOGS 2nd Annual Meeting 2005 June 20-24, 2005 — Suntec, Singapore

25. “EUV observations of sungrazing comets with the SOHO/UVCS instrument” THANK YOU! AOGS 2nd Annual Meeting 2005 June 20-24, 2005 — Suntec, Singapore

26. “EUV observations of sungrazing comets with the SOHO/UVCS instrument” AOGS 2nd Annual Meeting 2005 June 20-24, 2005 — Suntec, Singapore