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206_cetri,_Largo_E

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206_cetri,_Largo_E

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     A?very?faint?core-collapse?supernova?in?M85

     Arising?from:?S.?R.?Kulkarni?et?al.?Nature?447,?458?C460?(2007)

    A.?Pastorello1,?M.?Della?Valle2,3,4,?S.?J.?Smartt1?,?L.?Zampieri5,?S.?Benetti4,5,?E.?

    Cappellaro4,5,?P.?Mazzali6,7,?F.?Patat8,?S.?Spiro1,5,?M.?Turatto4,5,?S.?Valenti8,9

     e-mail:a.pastorello@qub.ac.uk

     1

    Astrophysics?Research?Centre,?School?of?Mathematics?and?Physics,?Queen??s?University ?

     Belfast,?Belfast?BT7?1NN,?United?Kingdom

     2?

    INAF?Osservatorio?Astronomico?di?Arcetri,?Largo?E.?Fermi?5,?I-50125?Firenze,?Italy????

    International?Center?for?Relativistic?Astrophysics?Network,?Piazzale?della?Repubblica ?

     3?

     10,?I-65122?Pescara,?Italy

     4?

    Kavli?Institute?for?Theoretical?Physics,?UC?Santa?Barbara,?California?93106,?USA

    INAF?Osservatorio?Astronomico?di?Padova,?Vicolo?dell'?Osservatorio?5,?I-35122 ?

     5?

     Padova,?Italy

     6

    Max-Planck-Institut?f??r?Astrophysik,?Karl-Schwarzschild-Str.?1,?D-85741?Garching?bei ?

     M??nchen,?Germany

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    INAF?Osservatorio?Astronomico?di?Trieste,?Via?Tiepolo?11,?I-34131?Trieste,?Italy ?European?Southern?Observatory?(ESO),?Karl-Schwarzschild-Str.?2,?D-85748?Garching ?

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     bei?M??nchen,?Germany

     9?

     Dipartimento?di?Fisica,?Universit?? di Ferrara, via del Paradiso 12, I-44100 Ferrara,

     Italy

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    An?anomalous?transient?in?the?early?Hubble-type?(S0)?galaxy?Messier?85?(M85)?in?

    the?Virgo?cluster?was?discovered?by?Kulkarni?et?al.1?on?7?January?2006?that?had?

    very?low?luminosity?(peak?absolute?R-band?magnitude?MR?of?about??12)?that?was?

    constant?over?more?than?80?days,?red?colour?and?narrow?spectral?lines,?which?seem?

    inconsistent?with?those?observed?in?any?known?class?of?transient?events.?Kulkarni?

    et?al.1?suggest?an?exotic?stellar?merger?as?the?possible?origin.?An?alternative?

    explanation?is?that?the?transient?in?M85?was?a?type?II-plateau?supernova?of?

    extremely?low?luminosity,?exploding?in?a?lenticular?galaxy?with?residual?star-

    forming?activity.?This?intriguing?transient?might?be?the?faintest?supernova?that?has?

    ever?been?discovered. ???????Kulkarni?et?al.1?suggest?that?this?event?(labelled?M85?OT2006-1)?was?an?anomalous?

    luminous?red?nova?possibly?generated?in?a?stellar?merger.??An?overall?similarity?with?

    other?transients?(M31?RV,?V4332?Sgr?and?V838And)?was?also?proposed2,?in?spite?of?the?

    fact?that?M85?OT2006-1?is??significantly?brighter?(by?two?to?three?magnitudes).?

    We?propose?an?alternative?scenario?in?which?M85?OT2006-1?is?an?extreme?case?of?a?

    low-luminosity?type?II-plateau?supernova?(LL-SNIIP)3,4,??about?1.5?magnitudes?fainter?

    than?the?faintest?LL-SNIIP?discovered?so?far?(SN1999br)4,5.?? ?????????

    The?morphology?of?its?light?curve1?is?very?similar?to?that?of?a?LL-SNIIP,?in?terms?of?

    the?plateau?duration?(typically?80-110??days)?and?the?low?peak?lum

inosity?(-15?<MV<-13,?

    Refs.?4-6?and?Fig.?1).?The?spectra?of??M85?OT2006-1?show?a?red?continuum?and?very?

    narrow?lines?(H?I,?Ca?II,?Ba?II,?Fe?II,?and?possibly?other?species,?Fig.?2)?which?are?

    commonly?observed?in?LL-SNeIIP?during?the?hydrogen?envelope?recombination?

    phase3-6.?Some?features?show?P-Cygni?profiles,?others?a?double-component?in?emission.?

    In?particular,?H?Á?shows?a?very?narrow?line??(v??Ö

    350?km?s-1,?Ref.?1)?on?the?top?of?a? broader?(v??Ö

    800?km?s-1?)?component?(Fig.?2,?insert?b)??which?is?consistent?with?the?

    typical?expansion?velocities?of?known?LL-SNeIIP4.?The?properties?of?this?subclass?of?

    SNe?are?explained?by?invoking?low?explosion?energies?(<<?1051?ergs)?and?very?small?

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    amounts?of?ejected?radioactive?56Ni?(<<?10-2?M?,?where?M??is?the?mass?of?the?Sun;?Ref.?

    4).? ????????????Core-collapse?supernovae?(CC-SNe)?are?normally?observed?in?spirals,?while?

    M85??is?a?lenticular?galaxy?(S0).?However,??traces?of?recent?episodes?of?star?formation?

    are?observed?in?many?local?early?type-galaxies,?in?the?form?of?faint?nebular?emission?

    lines,?tidal?tails,?dust?lines,?H?I?gas,?blue?colour?and?radio?emission.?Hence?the?fact?that?

    M85?OT2006-1?occurred?in?a?S0?galaxy?does?not?categorically?exclude?a?relatively?

    massive?progenitor?star.??It?has?been?suggested?that?moderate-mass?stars?(8-12M?)?can?

    explode?as?LL-SNeIIP?7-10,?and?it?is?certainly?possible?that?E/S0?galaxies?host?stellar?

    populations?of?such?masses.?Indeed,?about?a?dozen?CC-SNe?have?been?discovered?in?S0?

    galaxies?in?the?past?five?years11.?This?might?well?be?the?case?for?M85,?which?has?a?non- negligible?radio?flux?(L??Ö

    1027?erg?s-1?Hz-1),?placing?this?galaxy?in?the?faint?tail?of?the? radio?galaxies12.?Moreover,?the?strong?nuclear?H?Á?and?the?presence?of?an?inner?blue?ring?

    in?M85?are?indicative?that?recent?(minor)?episodes?of?star?formation?occurred13.????? ????

    One?of?the?arguments?advanced?by?Kulkarni?et?al.1?against?a?massive?star?origin?

    for?M85?OT2006-1?was?that?no?star?was?visible?in?deep?pre-explosion?Hubble?Space?

    Telescope?images.?However?the?detection?limit?of?mF850LP??=?24.7?(as?quoted?by?Kulkarni?

    et?al.1)?is?not?sufficient?to?exclude?a?moderately?massive?precursor.?If?it?was?a?type?K?to?

    M?supergiant,?after?adopting?the?assumptions?on?the?distance?and?the?reddening?by? Kulkarni?et?al.1,?a?luminosity?limit?of??logL/L??Ö 4.8?is?obtained.?Using?current?stellar?

    evolutionary?tracks?for?an?isolated?red?supergiant,?we?obtain?an?upper?mass?limit?of?

    15M?.?The?deep?detection?limit?in?the?blue?filter?(mF475W?=?26.8)?can?be?used?to?restrict?

    blue?supergiants?(B-type,?as?in?the?case?of?SN1987A)?to?less?than?about?12M?.?Small? star?clusters?of?initial?mass?about?300M? (obtained?using?the?stellar?population?models?

    of?Starburst99;?Ref.?14)?would?be?undetected?with?these?magnitude?limits.?Assuming?a?

    typical?initial?mass?function?in?such?a?cluster,?one?could?quite?reasonably?expect?a?few?

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    stars?above?the?limit?of?8M?.?In?light?of?all?of?this,?we?suggest?a?plausible?physical?

    interpretation?in?which?the?M85?transient?is?an?extremely?faint?CC-SN,?characterized?by?

    a?very?low?explosion?energy?(5-10?Á1049?ergs),?a?small?amount?of?56Ni?(<<?10-3?M?)?and?a? total?ejected?mass?of?6-9M?.?

     1.

    Kulkarni,?S.?R.?et?al.??An?unusually?brilliant?transient?in?the?galaxy?Messier?85.? Nature?447,?458-460?(2007) 2.

    Rau,?A.,?Kulkarni,?S.?R.,?Ofek,?E.?O.,?Yan,?L.?Spitzer?Observations?of?the?New?

    Luminous?Red?Nova?M85?OT2006-1.?Astrophys.?J.?659,?1536-1540?(2007) 3.

    Turatto,?M.?et?al.??The?Peculiar?Type?II?Supernova?1997D:?A?Case?for?a?Very? Low?56Ni?Mass.?Astrophys.?J.?498,?L129-L133?(1998) 4. Pastorello,?A.?et?al.??Low-luminosity?Type?II?supernovae:?spectroscopic?and?

    photometric?evolution.?Mon.?Not.?R.?Astron.?Soc.?347,?74-94?(2004) 5.

    Zampieri,?L.?et?al.?Peculiar,?low-luminosity?Type?II?supernovae:?low-energy?

    explosions?in?massive?progenitors??Mon.?Not.?R.?Astron.?Soc.?338,?711-716? (2003) 6.

    Pastorello,?A.?et?al.??SN?2005cs?in?M51?-?I.?The?first?month?of?evolution?of?a?

    subluminous?SN?II?plateau.?Mon.?Not.?R.?Astron.?Soc.?370,?1752-1762?(2006) 7.

    Chugai,?N.?N.,?Utrobin,?V.?P.?The?nature?of?SN?1997D:?low-mass?progenitor?and? weak?explosion.?Astron.?Astroph.?354,?557-566?(2000) 8. Maund,?J.?R.,?Smartt,?S.?J.,?Danziger,?I.?J.?The?progenitor?of?SN?2005cs?in?the?

    Whirlpool?Galaxy.?Mon.?Not.?R.?Astron.?Soc.?364,?L33-L37?(2005)?

     5

     9.

    Li,?W.?et?al.?Identification?of?the?Red?Supergiant?Progenitor?of?Supernova?

    2005cs:?Do?the?Progenitors?of?Type?II-P?Supernovae?Have?Low?Mass?? Astrophys.?J.?641,?1060-1070?(2006) 10.

    Kitaura,?F.?S.,?Janka,?H.-Th.,?Hillebrandt,?W.?Explosions?of?O-Ne-Mg?cores,?the?

    Crab?supernova,?and?subluminous?type?II-P?supernovae.?Astron.?Astroph.?450,? 345-350?(2006) 11.

    The?Padova-Asiago?Supernova?Catalogue;?http://web.pd.astro.it/supern/

    12. ?Della?Valle,?M.,?Panagia,?N.,?Padovani,?P.,?Cappellaro,?E.,?Mannucci,?F.,?Turatto,?

    M.?Why?Are?Radio?Galaxies?Prolific?Producers?of?Type?Ia?Supernovae?? Astrophys.?J.?629,?750-756?(2005) 13.

    Fisher,?D.?Kinematic?Profiles?of?SO?Galaxies.?Astron.?J.?113,?950-974?(1997)

    14. ?Leitherer,?C.?Starburst99:?Synthesis?Models?for?Galaxies?with?Active?Star? Formation.?Astrophys.?J.?Supp.?Ser.?123,?3-40?(1999) 15.

    Tonry,?J.?L.?et?al.?The?SBF?Survey?of?Galaxy?Distances.?IV.?SBF?Magnitudes,?

    Colors,?and?Distances.?Astrophys.?J.?546,?681-693?(2001)?

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    Figure?1?|?Absolute?R-band?magnitude?light?curves?for?M85?OT2006-1?and?

    LL-SNeIIP.?The?data?for?the?SN?sample,?adopted?explosion?epochs,?interstellar?

    extinction?and?distance?moduli?(calibrated?with?H0?=?72?km?s-1?Mpc

-1)?are?from?

    Refs.?4?and?6.?Unpublished?data?of?the?faint?SNIIP?2003Z?,?with?distance?

    modulus?(m?-?M)?=?32.64?and?interstellar?extinction?E(B?-?V)?=?0.04,?are?also?

    displayed.?For?M85?OT2006-1,?we?make?use?of?a?direct?estimate?of?the?surface?

    brightness?fluctuation?distance?to?M85?(Ref.?15),?giving?(m?-?M)?=?31.33??À?0.14?

    (18.5??À?1.2?Mpc),?adopting?the?same?interstellar?absorption?estimate?(AR?=?0.4)?

    as?in?Kulkarni?et?al.1.?With?an?average?plateau?magnitude?R?=?19.65,?we?obtain?

    an?absolute?magnitude?of?MR?=?-12.1.?LL-SNeIIP?cover?an?extended?range?of?

    luminosity,?and?thus?the?light?curve?of?M85?OT2006-1?(considering?the?

    photometric?errors?from?Kulkarni?et?al.1)?is?consistent?with?that?of?an?extremely? faint?SNIIP.?

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    Figure?2?|?Spectral?properties?of?M85?OT2006-1?and?LL-SNeIIP.?The? spectrum?of?M85?OT2006-1?obtained?on?24?February?2006?(from?Ref.?1,?kindly?

    provided?by?S.?R.?Kulkarni?and?A.?Rau,?de-reddened?by?AR?=?0.4?magnitudes)?is?

    compared?with?those?of?SN1999br?and?SN1999eu?observed?at?the?end?of?the?

    plateau4.?The?position?of?the?H?Balmer?lines,?Ba?II??Ë6497?and?the?near?infrared?

    Ca?II?triplet?are?marked?by?vertical?dashed?lines.?These?lines?are?visible?in?the?

    spectrum?of?the?transient,?while?there?is?no?clear?evidence?of?molecular?TiO?

    bands,?characterising?spectra?of?red?giants?and?the?peculiar?luminous?red?novae?

    proposed?in?refs?1?and?2.??F?is?the?flux?(in?erg?s-1)?scaled?to?an?arbitrary?constant.?

    The?axis?units?are?the?same?for?a?and?b.?a,??the?region?of?the?P-Cygni?Ba?II?

    ?Ë6497?+?H?Á?lines??in?SN1999eu?and?M85?OT2006-1.?b,?Three-component?fit? (broader?P-Cygni?feature?with?Vexp??Ö

    700-800?km?s-1,?plus?narrow?emission)?to?the?

    H?Á?profile.??The?red?line?shows?the?level?of?the?continuum,?the?g

reen?lines?show?

    separately?the?three?components?used?to?fit?the?line,?and?the?dotted?line?is?the? resulting?three-component?fit. ?

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