High-resolution X-ray spectra are a unique tool to investigate the accretion process in young stars. In fact X-rays allow to investigate the accretion-shock region, where the infalling material is heated by strong shocks due to the impact with the denser stellar atmosphere. Here we show for the first time that it is possible to constrain the velocity of the accretion stream by measuring the Doppler shift of the emitted X-rays. To this aim we analyzed the deep Chandra/HETGS observation of the accreting young star TW Hya. We selected a sample of emission lines free from significant blends, fitted them with gaussian profiles, computed the radial velocity corresponding to each line, and averaged these velocities to obtain an accurate estimate of the global velocity of the X-ray emitting plasma. After correcting for Earth's motion, we compared this observed velocity with the photospheric radial velocity. In order to check this procedure we applied the same technique to other Chandra/HETGS spectra of single stars, whose X-rays are due only to coronal plasma. While spectra of pure coronal sources provide Doppler shifts in agreement with the known stellar radial velocity, we found that the X-ray spectrum of TW Hya is red-shifted by ~30-40 km/s with respect to the stellar photosphere. This proves that the X-ray emitting plasma on TW Hya is moving with respect to the stellar surface, definitively confirming that it originates in the accretion-shock region. The observed velocity suggests that the base of the accretion region is located at low latitudes of the stellar surface

Argiroffi, Costanza

BONITO, Rosaria

Miceli, Marco

ORLANDO, Salvatore

Peres, Giovanni

OA@INAF - Istituto Nazionale di Astrofisica

2015Publication Year2020-03-23T16:00:57ZAcceptance in OA@INAFAccretion in young stars: measure of the stream velocity of TW Hya from the X-ray Doppler shiftTitleArgiroffi, Costanza; BONITO, Rosaria; ORLANDO, Salvatore; Miceli, Marco; Peres, GiovanniAuthorshttp://hdl.handle.net/20.500.12386/23470HandleTW	  Hya	  ξ	  Uma	  ACCRETION IN YOUNG STARS:!MEASURE OF THE STREAM VELOCITY OF TW HYA!FROM THE X-RAY DOPPLER SHIFTC. Argiroffi1,2, R. Bonito1,2, S. Orlando2, M. Miceli1,2, G. Peres1,2(1) Dip. di Fisica e Chimica – Universita’ degli Studi di Palermo, Italy (email: argi@astropa.unipa.it);(2) INAF – Osservatorio Astronomico di Palermo, Italy.CONCLUSIONS AND FUTURE PERSPECTIVES WITH ATHENAREFERENCES5. METHODTo	   measure	   the	   Doppler	   shi0	   in	   X-­‐ray	   spectra	   gathered	   with	   Chandra/HETGS	  we	  have:	  •  selected	  a	  sample	  of	  strong	  and	  isolated	  emission	  lines	  (Table	  1),	  •  fiGed	   each	   line	   individually,	   adopHng	   a	   gaussan	   profile,	   to	   determine	  the	  centroid	  posiHon	  (two	  examples	  are	  reported	  in	  Fig.	  2),	  •  determined	  the	  velocity	  corresponding	  to	  the	  displacement	  of	  each	  line	  with	  respect	  to	  its	  rest	  wavelength,	  •  computed	  the	  weighted	  mean	  of	  these	  velociHes	  (indicated	  with	  vX),	  •  evaluated	  the	  Earth	  velocity	  toward	  the	  target	  (indicated	  with	  vE),	  •  compared	  vX+vE	  (i.e.	  the	  X-­‐ray	  plasma	  radial	  velocity	  with	  respect	  to	  the	  barycentric	   reference	   system)	  with	   the	  predicted	   radial	   velocity	  of	   the	  star	  (indicated	  with	  v0).	  6. REFERENCE STAR SELECTIONTo	   test	   the	   reliability	   of	   our	  method	  we	   have	   applied	   it	   also	   to	   low-­‐mass	  stars	  that:	  •  have	   been	   observed	  with	  Chandra/HETGS	   providing	   spectra	  with	   high	  signal	  to	  noise	  raHo,	  •  are	  not	  affected	  by	  orbital	  moHon	  with	  high	  radial	  velociHes,	  •  have	  X-­‐ray	  emission	  due	  only	  to	  coronal	  plasma,	  since	  coronal	  plasma	  is	  not	   expected	   to	   have	   bulk	   moHons	   with	   respect	   to	   the	   stellar	  photosphere.	  All	   the	  stars,	  and	  the	  corresponding	  Chandra/HETGS	  spectra	  analyzed,	  are	  listed	  in	  Table	  2.	  	  7. RESULTSThe	   radial	   velociHes	   measured	   from	   the	   X-­‐ray	   spectra	   (vX)	   and	   their	  comparison	  with	  the	  expected	  photospheric	  velociHes	  are	  reported	  in	  Table	  2,	  and	  ploGed	  in	  Fig.	  3.	  From	  these	  values	  we	  find	  that:	  •  the	   X-­‐ray	   emiRng	   plasma	   on	   TW	  Hya	   shows	   a	   significant	   redshi0	  with	  respect	   to	   the	   stellar	   surface,	   corresponding	   to	  a	   velocity	  of	  36.5	  ±	  4.7	  km	  s-­‐1;	  •  all	  the	  other	  stars	  show	  X-­‐ray	  emiRng	  plasma	  velociHes	  compaHble	  with	  that	  of	  the	  stellar	  photosphere,	  validaHng	  the	  adopted	  method.	  Figure	  4	  reports,	  as	  an	  example,	  the	  comparinson	  of	  the	  O	  VIII	  line	  profiles	  of	  all	  the	  targets.	  	  8. THE ACCRETION ONTO TW HYAWe	  have	  measured	  for	  the	  first	  Hme	  the	  radial	  velocity	  of	  the	  X-­‐ray	  emiRng	  plasma	  in	  a	  CTTS.	  In	  parHcular	  we	  have	  found	   that	   the	   plasma	   at	   a	   few	   MK	   on	   TW	   Hya	   is	   redshi0ed	   of	   36.5	   ±	   4.7	   km	   s-­‐1	   with	   respect	   to	   the	   stellar	  photosphere.	  We	  can	  deduce	  that:	  •  so0	  X-­‐rays	  in	  CTTS	  are	  indeed	  produced	  in	  the	  post	  shock	  region;	  •  for	  TW	  Hya	  X-­‐ray	  and	  UV	  lines	  (in	  parHcular	  the	  narrow	  component	  of	  the	  C	  IV	  line,	  Ardila	  et	  al.	  2013)	  provide	  very	  similar	  radial	  velociHes,	  indicaHng	  that	  the	  plasma	  components	  at	  106	  K	  and	  that	  at	  105	  K	  are	  located	  in	  the	  same	  structure,	  i.e.	  the	  post-­‐shock	  region	  at	  the	  base	  of	  the	  accreHon	  stream;	  •  the	  post-­‐shock	  plasma	  is	  expected	  to	  have	  a	  velocity	  vpost	  of	  ≈	  100	  km	  s-­‐1	  (a	  minimum	  value	  of	  vpre	  is	  needed	  to	  have	  a	  post	  shock	  hot	  enough	  to	  radiate	  in	  X-­‐rays,	  Sacco	  et	  al.	  2010);	  then,	  since	  TW	  Hya	  is	  observed	  from	  the	  pole,	  an	  observed	  radial	  velocity	  for	  the	  post	  shock	  of	  ≈	  35	  km	  s-­‐1	  indicates	  that	  the	  base	  of	  the	  accreHon	  stream	  is	  located	  at	  low	  laHtudes	  on	  the	  stellar	  surface.	  Table	  2.	  VelociHes	  measured	  from	  the	  X-­‐ray	  emission	  lines,	  and	  velociHes	  expected.	  Star	   Chandra	   vX	   vE	   v0	   (vX	  +	  vE	  )-­‐	  v0	   n	  σ	  Obs	  Id	   (km	  s-­‐1)	   (km	  s-­‐1)	   (km	  s-­‐1)	   (km	  s-­‐1)	  TZ	  Crb	   15	   −5.0	  ±	  3.2	   −10.3	   −12.3	   −3.0	  ±	  3.2	   0.9	  EV	  Lac	  (1)	   1885	   −9.0	  ±	  5.0	   	  2.7	   0.4	   −6.7	  ±	  5.0	   1.3	  EV	  Lac	  (2)	   10679	   8.7	  ±	  6.2	   	  −4.8	   0.4	   3.4	  ±	  6.2	   0.5	  AU	  Mic	   17	   21.3	  ±	  6.0	   −28.2	   −4.5	   −2.4	  ±	  6.0	   0.4	  AD	  Leo	   2570	   43.4	  ±	  6.9	   −28.5	   12.5	   2.4	  ±	  6.9	   0.3	  TW	  Hya	   7435+7436+7437+7438	   38.3	  ±	  4.7	   11.6	   13.4	   36.5	  ±	  4.7	   7.8	  λ	  And	   609	   21.7	  ±	  4.2	   −20.2	   6.8	   −5.3	  ±	  4.2	   1.3	  ξ	  UMa	   1894	   −52.7	  ±	  4.4	   23.7	   −18.2	   −10.8	  ±	  4.4	   2.4	  24	  UMa	   2564+3471	   −1.9	  ±	  9.7	   −17.6	   −27.0	   7.5	  ±	  9.7	   0.8	  9. ACCRETION IN YOUNG STARS WITH ATHENAThe	  extremely	  larger	  effecHve	  area	  of	  ATHENA/X-­‐IFU,	  with	  respect	  to	  the	  Chandra	  and	  XMM	  graHngs,	  together	  with	  a	  spectral	  resoluHon	  of	  1.5	  eV	  in	  the	  so0	  X-­‐ray	  band,	  will	  allow	  to:	  •  enormously	  increase	  the	  number	  of	  CTTS	  for	  which	  high	  resoluHon	  X-­‐ray	  spectra	  can	  be	  gathered;	  up	  to	  now	  only	  a	   few	  CTTS	  have	  been	   invesHgated	  with	  high	  resoluHon	  X-­‐ray	  spectroscopy,	  with	  ATHENA	   it	  will	  be	  possible	   to	  invesHgate	  accreHng	  stars	  at	  different	  ages,	  different	  accreHon	  rates,	  different	  accreHon	  geometries;	  •  invesHgate	  accreHon	  on	  very	  short	  Hme	  scales,	   i.e.	  down	  to	  1	  ks;	  up	   to	  now	  CTTS	  X-­‐ray	  spectra	  are	   integrated	  over	  exposures	  of	  ≈100	  ks,	  while	  instead	  monitoring	  in	  other	  wavelength	  bands	  have	  proved	  that	  accreHon	  varies	  on	  significantly	  shorter	  Hme	  scales.	  One	  of	  the	  goals	  for	  the	  X-­‐IFU	  instrument	  is	  the	  measurement	  of	  bulk	  velociHes,	  reaching	  ≈20	  km	  s-­‐1	  as	  the	  minimum	  error	  from	  each	  individual	  spectral	  line.	  This	  resoluHon	  will	  allow	  to:	  	  •  measure	   the	   radial	   velocity	   of	   the	   post-­‐shock	   plasma	   for	   several	   CTTS,	   exploring	   intrinsic	   and	   rotaHonally	  modulated	  variaHons	  of	  vrad.	  Up	  to	  now	  TW	  Hya	  is	  the	  only	  CTTS	  for	  which	  the	  radial	  velocity	  measurements	  in	  X-­‐rays	  is	  possible.	  	  1. ACCRETION IN LOW MASS STARSYoung	   low-­‐mass	   stars,	   during	   their	   iniHal	   evoluHonary	   phases,	   accrete	  material	  from	  their	  circumstellar	  disk.	  These	  stars	  are	  defined	  as	  Classical	  T	  Tauri	  Stars	  (CTTS).	  The	  accreHon	  process	  is	  regulated	  by	  the	  intense	  stellar	  magneHc	  fields,	  that	  disrupt	  the	  inner	  disk	  at	  a	  few	  stellar	  radii,	  and	  guide	  accreHng	  material	  in	  a	  free	  fall	  toward	  the	  central	  star	  (Fig.	  1).	  At	  the	  base	  of	  the	  stream,	  infalling	  material	  reaches	  velociHes	  of	  ≈	  300	  −	  500	  km	  s-­‐1,	  and	  a	   strong	   shock	   forms	   because	   of	   the	   impact	   with	   the	   denser	   stellar	  atmosphere	  	  (Königl	  1991)	  .	  	  2. X-RAYS FROM ACCRETIONHigh-­‐resoluHon	   X-­‐ray	   spectra	   of	   CTTS	   indicated	   that	   in	   these	   sources	   the	  plasma	   at	   a	   few	  MK	   is	   at	   high	   density	   (Kastner	   et	   al.	   2002),	   at	   odds	  with	  other	  non-­‐accreHng	  low-­‐mass	  stars	  (e.g.	  Testa	  et	  al.	  2004),	  where	  X-­‐rays	  are	  emiGed	  by	  coronal	  plasma.	  This	  high	  density	  has	  been	  considered	  the	  main	  evidence	   that	   the	   cool	   plasma	   component	   in	   CTTS	   is	   not	   coronal	   plasma,	  but	  it	   is	  material	  heated	  in	  the	  accreHon	  shock.	  In	  fact	  the	  infall	  pre-­‐shock	  velociHes,	  vpre,	  are	  expected	  be	  so	  high	  to	  produce	  a	  post	  shock	  plasma	  at	  temperature	  of	  a	  few	  MK.	  The	  post	  shock	  plasma	  is	  also	  expected	  to	  move	  with	  a	  post	  shock	  velocity	  vpost	  =	  vpre/4	  (Fig.	  1).	  	  3. THE CHANDRA AND XMM ERAHigh-­‐resoluHon	  X-­‐ray	  spectra	  are	  proved	  to	  be	  a	  unique	  tool	  to	  invesHgate	  the	   accreHon	   process	   in	   CTTS:	   so0	   X-­‐rays	   directly	   probe	   the	   base	   of	   the	  acccreHon	  stream,	  allowing	  to	  infer	  the	  physical	  condiHons	  of	  the	  accreHng	  material,	   like	  density	  and	  chemical	   composiHon.	  The	  effecHve	  area	  of	   the	  Chandra	  and	  XMM	  graHngs	  allowed	  to	  gather	  high-­‐resoluHon	  X-­‐ray	  spectra	  only	  for	  a	  very	  few	  CTTS,	  leaving	  important	  issues	  to	  be	  addressed,	  like	  the	  mass	  accreHon	  rate	  esHmates	  (Curran	  et	  al.	  2011),	  or	  the	  possibility	  to	  have	  an	  accreHon	  feed	  corona	  (Brickhouse	  et	  al.	  2010).	  	  4. AIMS OF THIS WORKHere	  we	  show	  for	  the	  first	  Hme	  the	  esHmate	  of	  the	  radial	  velocity	  of	  the	  X-­‐ray	  emiRng	  plasma	  in	  CTTS	  by	  measuring	  the	  line	  Doppler	  shi0	  in	  the	  X-­‐ray	  band.	  A	  redshi0	  with	  respect	  to	  the	  stellar	  photosphere,	  expected	  for	  the	  plasma	  located	  in	  the	  post-­‐shock	  region	  (Fig.	  1),	  would	  definiHvely	  indicate	  that	  this	  plasma	  is	  located	  in	  the	  accreHon-­‐shock	  region.	  To	  this	  aim	  we	  analyzed	  the	  deep	  (500	  ks)	  Chandra/HETGS	  observaHon	  of	  the	  CTTS	  TW	  Hya.  TW	  Hya	  is	  a	  single	  0.7	  M¤	  star,	  8	  Myr	  old,	  with	  an	  X-­‐ray	  luminosity	  of	  1.4×1030	  erg	  s-­‐1,	  and	  with	  an	  accreHon	  rate	  of	  ≈	  10-­‐9	  M¤	  yr-­‐1.	  This	  star,	  located	  at	  55	  pc,	  is	  observed	  almost	  pole	  on.	  Ardila,	  D.	  R.,	  Herczeg,	  G.	  J.,	  Gregory,	  S.	  G.,	  et	  al.	  2013,	  ApJS,	  207,	  1	  Brickhouse,	  N.	  S.,	  Cranmer,	  S.	  R.,	  Dupree,	  A.	  K.,	  	  et	  al.	  2010,	  ApJ,	  710,	  1835	  Curran,	  R.	  L.,	  Argiroffi,	  C.,	  Sacco,	  G.	  G.,	  et	  al.	  2011,	  A&A,	  526,	  A104	  Kastner,	  J.	  H.,	  Huenemoerder,	  D.	  P.,	  Schulz,	  N.	  S.	  et	  al.	  2002,	  ApJ,	  567,	  434	  Königl,	  A.	  1991,	  ApJ	  370,	  L39	  Sacco,	  G.	  G.,	  Orlando,	  S.,	  Argiroffi,	  C.,	  et	  al.	  2010	  A&A,	  522,	  55	  Testa,	  P.,	  Drake,	  J.	  J.,	  &	  Peres,	  G.	  2004,	  ApJ,	  617,	  508	  Figure	  1.	  SchemaHc	  view	  of	  the	  accreHon	  stream	  in	  a	  CTTS.	  The	  yellow	  area	  at	  the	  base	   of	   the	   stream	   indicates	   the	   hot	   plasma	   heated	   in	   the	   accreHon	   shock	   and	  emiRng	  X-­‐rays.	  Table	  1.	  Line	  selected	  for	  the	  Doppler	  shi0	  measurements.	  wavelength	   Ion	   Tmax	  (Å)	   (MK)	  10.24	   Ne	  X	   6	  13.70	   Ne	  IX	   4	  16.01	   O	  VIII	   3	  18.97	   O	  VIII	   3	  21.60	   O	  VII	   2	  Figure	   3.	   Comparison	   between	   the	   observed	   radial	   velocity	   in	   X-­‐rays,	   vX	   +	   vE,	  (velocity	  with	  respect	  to	  the	  barycentric	  reference	  system,	  red	  diamonds),	  and	  the	  radial	  photospheric	  velocity	  (blue	  diamonds)	  for	  each	  inspected	  star.	  Figure	  4.	  Observed	  profiles	  of	  the	  O	  VIII	  Lyα	  line,	  of	  all	  the	  X-­‐ray	  spectra	  analyzed,	  with	   respect	   to	   the	   stellar	   reference	   frame.	  All	   the	   inspected	   stars,	   but	   TW	  Hya,	  show	  no	  shi0	  with	  respect	  to	  the	  stellar	  frame.	  The	  redshi0	  of	  the	  TW	  Hya	  X-­‐ray	  spectrum	  is	  small	  but	  significant.	  Figure	  2.	  Observed	  spectra	  (black	  line)	  in	  the	  region	  of	  Ne	  IX	  line	  at	  13.70	  Å.	  Red	  curve	  indicates	  the	  best	  fit	  of	  the	  line.	  The	  two	  spectra,	  TW	  Hya	  in	  the	  upper	  panel	  and	  ξ	  Uma	  in	  the	  lower	  panel,	  represent	  the	  case	  of	  a	  red	  shi0ed	  and	  blue	  shi0ed	  line,	  respecHvely.	  The	  vX	  corresponding	  to	  these	  spectra	  are	  reported	  in	  Table	  2.	  DOPPLER SHIFT MEASUREMENTS WITH CHANDRA/HETGSCONTEXTvpre	  Vpost	  	  shock	  Poster	  presented	  at	  the:	  Exploring	  the	  Hot	  and	  Energe:c	  Universe:	  The	  first	  scien:fic	  conference	  dedicated	  to	  the	  Athena	  X-­‐ray	  observatory,	  Madrid,	  September	  2015	   C.	  Argiroffi,	  argi@astropa.unipa.it	  AccreAng	  low-­‐mass	  star	  AccreAon	  stream	  

Accretion in young stars: measure of the stream velocity of TW Hya from the X-ray Doppler shift

https://openaccess.inaf.it/bitstream/20.500.12386/23470/1/CArgiroffi_p.pdf

Accretion in young stars: measure of the stream velocity of TW Hya from the X-ray Doppler shift

Abstract

Similar works

Full text

Available Versions

OA@INAF - Istituto Nazionale di Astrofisica