“EUV observations of sungrazing comets with the SOHO/UVCS instrument” EUV OBSERVATIONS OF SUNGRAZING COMETS WITH THE SOHO/UVCS INSTRUMENT A. Bemporad 1, G. Poletto 2, J. C. Raymond 3 and S. Giordano 4 1 Astronomy and Space Science Dept. – University of Firenze – Firenze, Italy - Arcetri Astrophysical Observatory – Firenze, Italy 3 Harvard - Smithsonian Center for Astrophysics, Cambridge, MA, USA 4 INAF – Torino Astrophysical Observatory, Torino, Italy 2 INAF University of Florence Arcetri Astrophysical Observatory Harvard-Smithsonian CFA Torino Astrophysical Observatory AOGS 2 nd Annual Meeting 2005 June 20 -24, 2005 — Suntec, Singapore
“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 2 nd Annual Meeting 2005 June 20 -24, 2005 — Suntec, Singapore
“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 P 78 -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 2 nd Annual Meeting 2005 June 20 -24, 2005 — Suntec, Singapore
“EUV observations of sungrazing comets with the SOHO/UVCS instrument” THE LASCO (Large Angle Spectroscopic Coronagraph) INSTRUMENT ON SOHO C/1996 Y 1 LASCO/C 2: from 2. 0 to 6. 0 Rsun CCD 10242 - Pixel size: 11. 4” LASCO/C 3: from 3. 7 to 32 Rsun CCD 10242 - Pixel size: 56. 0” AOGS 2 nd Annual Meeting 2005 June 20 -24, 2005 — Suntec, Singapore
“EUV observations of sungrazing comets with the SOHO/UVCS instrument” THE UVCS (Ultra. Violet Coronagraph Spectrometer) INSTRUMENT ON SOHO UVCS: from 1. 4 to 10 Rsun - CCD 1024 X 360 Pixel size: 0. 0925Å spectral, 7” spatial AOGS 2 nd Annual Meeting 2005 June 20 -24, 2005 — Suntec, Singapore
“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… Year Comets # Obs. Date Comet Name 1996 1997 1998 1999 1 1 1 2 2000 4 2001 2002 1 2 2003 1 Dec 23 May 1 -2 Jun 1 May 20 Sep 17 Feb 10 Feb 29 Sep 9 -11 Oct 11 Feb 6 -7 May 14 Sep 18 Jan 27 -29 Comet Group C/1996 Y 1 SOHO-6 Kreutz C/1997 H 2 SOHO-8 None C/1998 K 10 SOHO-54 Kreutz C/1999 K 1 SOHO-63 Kreutz C/1999 S 1 SOHO-86 Kreutz C/2000 C 6 SOHO-104 Kreutz C/2000 D 1 SOHO-106 Kreutz 2 P/ENCKE None C/2000 T 1 SOHO-204 Kreutz C/2001 C 2 SOHO-294 Kreutz C/2002 J 8 SOHO-442 Kreutz C/2002 S 2 SOHO-517 Kreutz C/2002 X 5 Kudo-Fujikawa None • UVCS observed only 13 comets: 10 of these belong to the Kreutz Group. • Until now, only 3 works have been published. Published Yes No No Yes No No Yes AOGS 2 nd Annual Meeting 2005 June 20 -24, 2005 — Suntec, Singapore
“EUV observations of sungrazing comets with the SOHO/UVCS instrument” UVCS SUNGRAZER OBSERVATIONS: DISCUSSION PLAN 1. UVCS observations of sungrazers revealed a high cometary emission in the Hydrogen Lyman-α λ 1216Å spectral line. This implies an enhanced number of H atoms 2. Cometary H atoms may be created by different processes: from an analysis of the Lyα profile we can infer their origin 3. From the observed Lyα emission we derive an estimate for the cometary outgassing rate and the nucleus size 4. From the variation with the heliocentric distance of the outgassing rate we discuss possible fragmentation processes AOGS 2 nd Annual Meeting 2005 June 20 -24, 2005 — Suntec, Singapore
“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 λ 1025Å, also included in the UVCS spectra, and the Lyman-α spectral lines. At typical coronal temperatures (~106 K): collisional Lyβ/Lyα ~ 0. 13 Because of the small chromospheric ratio: radiative Lyβ/Lyα ~ 0. 001 -0. 002 In the UVCS observations of C/1996 Y 1, C/2000 C 6 and C/2001 C 2 the Lyβ line was absent Cometary Lyα emission only from Radiative excitation AOGS 2 nd Annual Meeting 2005 June 20 -24, 2005 — Suntec, Singapore
“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 H 2 O molecules • Charge exchange process between coronal p+ and H atoms from H 2 O 1. 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. H atoms from H 2 O photodissociation move with the comet 2. The momentum transfer in the charge exchange process between p+ and H atoms is very small (Mc. Clure 1966). Secondary H atoms have the coronal plasma velocity distribution AOGS 2 nd Annual Meeting 2005 June 20 -24, 2005 — Suntec, Singapore
“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 H 2 O 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… profile (from Uzzo et al. 2001) AOGS 2 nd Annual Meeting 2005 June 20 -24, 2005 — Suntec, Singapore
“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. H atoms responsible for the sungrazer emission form from the charge exhange between coronal p+ and H atoms from H 2 O (from Uzzo et al. 2001) AOGS 2 nd Annual Meeting 2005 June 20 -24, 2005 — Suntec, Singapore
“EUV observations of sungrazing comets with the SOHO/UVCS instrument” ESTIMATE OF SUNGRAZERS OUTGASSING RATE AND NUCLEUS SIZE 1. The comet leaves along its path a number of neutral H atoms Ncoma 2. 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 ). 2. Then, after the comet transit, the total number of Lyα counts 3. exponentially decays with time as exp(-t/ ion) , where ion is the ionizatio 4. rate (mainly by collisions with electrons, ion ∝ ne-1). From the observed (Lyα)counts vs. time curve we derive the cometary outgassing rate and the local plasma electron density ne AOGS 2 nd Annual Meeting 2005 (from Bemporad et al. 2005) June 20 -24, 2005 — Suntec, Singapore
“EUV observations of sungrazing comets with the SOHO/UVCS instrument” ESTIMATE OF SUNGRAZERS OUTGASSING RATE AND NUCLEUS SIZE 1. The comet leaves along its path a number of neutral H atoms Ncoma 2. 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 ). 2. Then, after the comet transit, the total number of Lyα counts 3. exponentially decays with time as exp(-t/ ion) , where ion is the ionizatio 4. rate (mainly by collisions with electrons, ion ∝ ne-1). Assuming a spherical nucleus and isotropic outgassing we estimate its equivalent radius From the observed (Lyα)counts vs. time curve we derive the cometary outgassing rate and the local plasma electron density ne AOGS 2 nd Annual Meeting 2005 June 20 -24, 2005 — Suntec, Singapore
“EUV observations of sungrazing comets with the SOHO/UVCS instrument” UVCS OBSERVATION OF SUNGRAZERS: RESULTS (LASCO running difference movies) Comet name QH 2 O (kg/s) r ( m) h ( Ro ) C/1996 Y 1 C/2000 C 6 20. 0 71. 8 140 34. 6 10. 5 820 a 58. 9 a 28. 5 b 3. 4 3. 0 5. 8 3. 4 2. 5 20. 3 7. 8 5. 4 6. 80 3. 26 4. 56 5. 71 6. 36 3. 60 4. 98 C/2001 C 2 UVCS observations revealed a “hidden mass” inconsistent with nominal mass 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). AOGS 2 nd Annual Meeting 2005 June 20 -24, 2005 — Suntec, Singapore
“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. 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. AOGS 2 nd Annual Meeting 2005 (from Sekanina 2003) June 20 -24, 2005 — Suntec, Singapore
“EUV observations of sungrazing comets with the SOHO/UVCS instrument” SUNGRAZING COMETS: FRAGMENTATION PROCESSES 3. (from Sekanina & Chodas 2004) 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 fragmentation, of the observed sungramentation zers from a single progenitor Close to the Sun, sungrazer fragmentation is more probable because of: a) High tidal stresses (∝r 2/d 3 ), in particular around the Roche limit (~ 3. 2 Ro) b) High thermal stresses, mainly on the nucleus surface stresses c) High sublimation flux can create an average surface pressure overcoming the tensile strength AOGS 2 nd Annual Meeting 2005 June 20 -24, 2005 — Suntec, Singapore
“EUV observations of sungrazing comets with the SOHO/UVCS instrument” SUNGRAZING COMETS: THE OBSERVED FRAGMENTATION OF C/2001 C 2 UVCS observations of C/2001 C 2 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 fragments least 2 more alternatives are possible: Tail 1 Tail 2 (from Bemporad et al. 2005) 1) 2 tails generated by a single object: however, at 5 Ro the nucleus is exposed to an extremely high solar flux (~ 2. 5· 109 ergs cm-2 s-1) → the whole surface should be active. 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 ~1 h later the comet shows AOGS 2 nd Annual Meeting 2005 only one tail at 3. 60 Ro. June 20 -24, 2005 — Suntec, Singapore
“EUV observations of sungrazing comets with the SOHO/UVCS instrument” SUNGRAZING COMETS: THE OBSERVED FRAGMENTATION OF C/2001 C 2 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”). This interpretation seems to be confirmed by the C/2001 C 2 lightcurve which shows a secondary brightnening below ~ 5. 3 Ro. AOGS 2 nd Annual Meeting 2005 June 20 -24, 2005 — Suntec, Singapore
“EUV observations of sungrazing comets with the SOHO/UVCS instrument” SUNGRAZING COMETS: BOW SHOCK (from http: //www. astro. uwo. ca/~jlandstr/planets/) 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 C 6 and C/2001 C 2 were immersed in a slow wind region; no significant plasma heating was observed from Lyα profiles. Sungrazer C/1996 Y 1 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, AOGS 2 nd Annual Meeting 2005 Raymond et al. 1998 inferred Vwind ~ 620 km/s June 20 -24, 2005 — Suntec, Singapore
“EUV observations of sungrazing comets with the SOHO/UVCS instrument” C/2002 S 2 SUNGRAZING COMET: WORK IN PROGRESS On September 18 -19, 2002 UVCS observed the sungrazer C/2002 S 2 at 4 heliocentric distances. UVCS observations of sungrazer C/2002 S 2 revealed the presence of 2 Lyα tails at 6. 84 Ro and possibly also at lower heights. LASCO/C 2 & C 3 movie (S. Giordano, private comunication) AOGS 2 nd Annual Meeting 2005 June 20 -24, 2005 — Suntec, Singapore
“EUV observations of sungrazing comets with the SOHO/UVCS instrument” C/2002 S 2 SUNGRAZING COMET: OPEN QUESTIONS The puzzling Lyα Doppler shift image revealed the 2 observed tails to be red- and blueshifted by ~ 100 km/s. These cannot be interpreted as the signature of 2 fragments. The observed curves for the Lyα counts vs. time show a slow increase, difficult to explain with the increase actual model. AOGS 2 nd Annual Meeting 2005 June 20 -24, 2005 — Suntec, Singapore
“EUV observations of sungrazing comets with the SOHO/UVCS instrument” C/2002 S 2 SUNGRAZING COMET: THE MODEL Actually the formulation of a simple model to explain the observed Lyα features is in progress. 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 AOGS 2 nd Annual Meeting 2005 June 20 -24, 2005 — Suntec, Singapore
“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 λ 1216Å spectral line. H atoms responsible for this emission are those formed with a charge exchange process between H atoms from H 2 O 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 2 nd Annual Meeting 2005 June 20 -24, 2005 — Suntec, Singapore
“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 C 2), the presence of sub-fragments, unresolved by LASCO coronagraphs, sub-fragments has been directly observed for the C/2001 C 2 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 S 2 sungrazer is actually a “work in progress”: a good model of the cometary tail could help us in order to explain the observed features. AOGS 2 nd Annual Meeting 2005 June 20 -24, 2005 — Suntec, Singapore
“EUV observations of sungrazing comets with the SOHO/UVCS instrument” AOGS 2 nd Annual Meeting 2005 June 20 -24, 2005 — Suntec, Singapore
“EUV observations of sungrazing comets with the SOHO/UVCS instrument” AOGS 2 nd Annual Meeting 2005 June 20 -24, 2005 — Suntec, Singapore
Скачать презентацию EUV observations of sungrazing comets with the SOHO UVCS
b143a989b074b9ce07c133812b3e9767.ppt