K2's Revolutionary Eye on White Dwarfs

Conference presentation, 25 min. November 2015: K2 Science Conference, Santa Barbara, CA, USA.

Transcript

1. K2’s Revolutionary Eye on White Dwarf Stars JJ Hermes Hubble Fellow, University of North Carolina University of Warwick
2. K1: 20 WDs observed,
3. Kepler K2
4. Kepler K2
8. Planets! Supernovae!
9. White Dwarfs: Simple Stars, Simple Evolution •  >80%  of  white  dwarfs  have  hydrogen-­‐‑dominated  atmosphere  (DA) •  Vast  majority  of  DAs  ~0.6    M¤   •  Simple  evolution:  just  cooling Balmer line fits à masses Tremblay+ 2011 MWD  =  1.4  M¤ MWD  =   0.4  M¤
10. White Dwarfs: Not Every One is So Simple DZ  (atmospheric  metals) 25-50% of white dwarfs are polluted with metals •  Metals  are  coming  from  remnant  planetary  systems! •  log(g)  =  8.0,  so  metals   should  quickly  sink   out  of  photosphere
11. •  Abundances:  debris  is  rocky  and  chemically  diverse,  like  solar   system  meteorites  (Gänsicke+12) •  Some  debris  is  rocky  &  water-­‐‑rich  (Farihi+13, Raddi+2015) •  Infrared  excesses  often  seen:  directly  detecting  debris  disks •  Exoplanet   compositions! The Scars of Tidally Disrupted Planetesimals
12. Vanderburg+ 2015 K2: First Transiting Planetesimals Around WDs •  This  WD  has   metal  pollution   and  an   IR  excess   (dust  disk) •  See  talk  by   Andrew   Vanderburg   tomorrow:  A   disintegrating   minor  planet   transiting  a   white  dwarf
13. Either Way to Supernovae Ia: Need a White Dwarf
14. K2: Dozens of Short-Period WD Binaries •  By  the  end  of  K2  we  will  get   orbital  periods  for  dozens  of   pre-­‐‑  Cataclysmic  Variables   (WD+dM  binaries) Rebassa-Mansergas, Hermes+ in prep. 99,860  K 0.59  Msun  WD Reflection off dM every 19.9 hr
16. A ‘typical’ white dwarf electron degenerate
17. (Astero)Seismology White  Dwarf Earth
18. Convection Drives White Dwarf Pulsations •  DA  (hydrogen   atmosphere)  WDs   pulsate  when  H  partially   ionized  (DAVs,  aka   ZZ  Cetis) g-modes—remarkably similar to the large-amplitude DAV pulsators (Winget et al. 1 The observed pulsating white dwarf stars lie in three strips in the H-R diagram, in Figure 3. The pulsating pre-white dwarf PG 1159 stars, the DOVs, around 7 170,000 K have the highest number of detected modes. The first class of pulsating 5.5 5.0 4.5 Planetary Nebula Main sequence DOV DBV DAV 4.0 3.5 3.0 log [T eff (K)] 4 2 0 –2 –4 log (L/L ) Figure 3 A 13-Gyr isochrone with z = 0.019 from Marigo et al. (2007), on which we have drawn the ob Annu. Rev. Astro. Astrophys. 2008.46:157-199. Downloaded f by University of Texas - Austin on 01/28/09. For Winget & Kepler 2008 Partial ionization zone can store & release energy
19. Winget & Kepler 2008 Fontaine & Brassard 2008 Fractional  Mass  Depth:  log  q  –  log  (1-­‐‑M(r)/MWD ) N2 Ll 2 “Propagation  Diagram” Core Surface Seeing Inside a WD Takeaway:  Chemical  transitions   cause  a  “bump”  in  N2,  and  thus  a   detectable  asteroseismic   signature p-modes σ2  >  Ll 2,  N2 g-modes σ2  <  Ll 2,  N2 convection
20. 1000 s 200 s 500 s
21. Kepler   12  May  2009  –   11  May  2013 K2   17  Jan  2014  –     Ongoing
22. GD  1212,  Hermes  et  al.  2014,  ApJ,  789,  85 The First K2 Pulsating White Dwarf Eng.  Run,  KP  =  13.3  mag,  revisit  in  Field  12
23. 14+ hr
24. §  Core Angular Momentum Evolution §  How Pulsations Relate to WD Temperature §  Outbursts?! What Has K2 Taught Us So Far?
25. White Dwarf Rotation Made Easy with Kepler •  Kp  =  18.0  mag •  1  month  Kepler  data •  Frequency  spliiings  yield   Prot  =  0.9  ±  0.3  d •  0.62  ±  0.05  M¤  WD:   ~2.2  M¤  (dA)  progenitor l  =  1  modes: m  =  +1 m  =  -­‐‑1 m  =  0
26. A New K2 View on Common-Envelope Evolution •  WD+dM  in  K2  Campaign  1:   SDSS  J1136+0409 SDSS SOAR VLT Hermes et al. 2015, MNRAS, 451, 1701
27. A New K2 View on Common-Envelope Evolution M-dwarf RV (VLT/FORS2) •  WD+dM  in  K2  Campaign  1:   SDSS  J1136+0409 WD atmospheric parameters (SOAR) Teff  =  12,330  ±  260  K log(g)  =  7.99  ±  0.06  (0.601  ±  0.036  M¤ ) SDSS SOAR VLT Hermes et al. 2015, MNRAS, 451, 1701 Porb  =  6.8976  hr
28. A New K2 View on Common-Envelope Evolution M-dwarf RV (VLT/FORS2) Porb  =  6.8976  hr (Model:  Doppler  beaming,  reflection,  ellipsoidal   variations  using  spectroscopic  parameters) •  WD+dM  in  K2  Campaign  1:   SDSS  J1136+0409 WD atmospheric parameters (SOAR) Teff  =  12,330  ±  260  K log(g)  =  7.99  ±  0.06  (0.601  ±  0.036  M¤ ) SDSS SOAR VLT Hermes et al. 2015, MNRAS, 451, 1701 Folded K2 light curve
29. A New K2 View on Common-Envelope Evolution 10500 11000 11500 12000 12500 White dwarf effective temperature (K) 0.54 0.56 0.58 0.60 0.62 0.64 0.66 0.68 0.70 White dwarf mass (Msun) 10500 11000 11500 12000 12500 White dwarf effective temperature (K) −5.6 −5.4 −5.2 −5.0 −4.8 −4.6 −4.4 Hydrogen layer mass (log MH/Mstar) 20 22 24 26 28 30 32 34 7 independent pulsation modes 1-­‐‑σ  Spectroscopic   Teff   /  mass Best   asteroseismic  fit Hermes et al. 2015, MNRAS, 451, 1701
30. A New K2 View on Common-Envelope Evolution Hermes et al. 2015, MNRAS, 451, 1701 J1136+0409  Prot :   2.49  ±  0.53  hr
31. A New K2 View on Common-Envelope Evolution 10 1 100 101 102 White Dwarf Rotation Period (hr) 0 1 2 3 4 5 6 N Non-magnetic CVs Pulsating white dwarfs J1136+0409 J1136+0409  Prot :   2.49  ±  0.53  hr ~Days ~Minutes •  No  isolated  WD  rotates  this  fast •  No  accretion  history  in  J1136+0409 •  RGB  core  evolution  influenced  by   common  envelope  ejection Hermes et al. 2015, MNRAS, 451, 1701
32. §  Core Angular Momentum Evolution §  How Pulsations Relate to WD Temperature §  Outbursts?! What Has K2 Taught Us So Far?
33. The Empirical DAV Instability Strip Today Gianninas+ 2011 Tremblay+ 2011 3D-­‐‑Corrected  Atmospheric  Parameters,  ML2/α  =  0.8 Known pulsating white dwarfs Non-variable white dwarfs
34. Convective Driving: WD Cools, Periods Increase
35. Convective Driving: WD Cools, Periods Increase Mukadam+ 2006 V. Van Grootel et al.: The instab Fig. 2. Structure of the envelope of our representative evolving 0.6 M DA white dwarf. The ordinate gives the fractional mass depth in loga rithmic units. The small dots define “isocontours” of opacity, and som Surface Core Base  of   convection  zone   deepens  as  WD  cools Van Grootel+ 2012
36. 1000 s 200 s 500 s
37. §  Core Angular Momentum Evolution §  How Pulsations Relate to WD Temperature §  Outbursts?! What Has K2 Taught Us So Far?
38. The First Kepler DAV Showed Something Funny 2 Bell et al. Fig. 1.— Representative sections of the Kepler light curve of KIC 4552982 in units of days since the start of observations. The top p shows the full Q11 light curve. The one-month shaded region in the top panel is expanded in the middle panel. The one-week sh region in the middle panel is expanded in the bottom panel. The solid line is the light curve smoothed with a 30-minute window. point-to-point scatter dominates the pulsation amplitudes in the light curve, so pulsations are not apparent to the eye. The dram increases in brightness are discussed in detail in Section 3. to medium-resolution spectra for the white dwarf and fit the Balmer line profiles to models to determine its val- tion rate. We summarize our findings and conclud Section 5. KIC 4552982: Bell+ 2015 3 months: 1 month: 1 week:
39. The First Kepler DAV Showed Something Funny 2 Bell et al. Fig. 1.— Representative sections of the Kepler light curve of KIC 4552982 in units of days since the start of observations. The top p shows the full Q11 light curve. The one-month shaded region in the top panel is expanded in the middle panel. The one-week sh region in the middle panel is expanded in the bottom panel. The solid line is the light curve smoothed with a 30-minute window. point-to-point scatter dominates the pulsation amplitudes in the light curve, so pulsations are not apparent to the eye. The dram increases in brightness are discussed in detail in Section 3. to medium-resolution spectra for the white dwarf and fit the Balmer line profiles to models to determine its val- tion rate. We summarize our findings and conclud Section 5. KIC 4552982: Bell+ 2015 3 months: 1 month: 1 week: e measured equivalent durations of the 186 outbursts at were recorded without interruption from gaps in the ta is displayed in Figure 4 and the continua used for e example outbursts are the dashed lines in Figure 3. e median outburst equivalent duration is 6.8 minutes he corresponding outburst is displayed in the second nel of Figure 3). Since the Kepler point-to-point scat- is 1.8% for this target, we are limited to detecting ly large outbursts by eye and so are undoubtedly in-
40. A Second Case of Outbursts in a Cool DAV PG 1149+057: Hermes et al. 2015, ApJ, 810, L5 1145.7 s, 998.1 s, 1052.8 s
41. A Second Case of Outbursts in a Cool DAV PG 1149+057: Hermes et al. 2015, ApJ, 810, L5 1145.7 s, 998.1 s, 1052.8 s
42. A Second Case of Outbursts in a Cool DAV PG 1149+057: Hermes et al. 2015, ApJ, 810, L5 •  No  companion  earlier  than  L3:   This  is  happening  on  the  white  dwarf SDSS image K2 pixels 11,000  K,  log(g)=8.0  model   (3σ  uncertainties  smaller  than  each  point)
43. (Aside: The Ecliptic is Full of Asteroids!) •  Always  check  the  target   pixels  if  you  see  a   blip  in  your  K2  light  curve Data  from  J1136+0409   (WD+dM  from  earlier)   h5p://barentsen.github.io/k2flix/ K2flix
44. A Second Case of Outbursts in a Cool DAV PG 1149+057: Hermes et al. 2015, ApJ, 810, L5 •  These  outbursts  are  essentially  rogue  waves  in  a  pulsating  star! •  The  pulsations  persist  in  outburst
45. A Second Case of Outbursts in a Cool DAV PG 1149+057: Hermes et al. 2015, ApJ, 810, L5 K2 Campaign 1 full light curve Prot  ≈  1.2  d
46. Pulsations Persist in Outburst •  White  dwarf  Teff  =  11,060  K •  é  14%  mean  flux    =    é  750  K •  é  >25%  flux    =    é  >1500  K Black line is 30-min running mean
47. A Second Case of Outbursts in a Cool DAV PG 1149+057: Hermes et al. 2015, ApJ, 810, L5 •  Pulsations  change  after  outburst
48. A Second Case of Outbursts in a Cool DAV PG 1149+057: Hermes et al. 2015, ApJ, 810, L5 (3-­‐‑day  sliding  window)
49. Potential Outburst Mechanisms in Cool DAVs •  Magnetism  unlikely:  τdynamical   only  a  few  s  for  WDs •  Nuclear  burning  unlikely:   T  <  106  K  at  τthermal  of  recurrence   timescale  (~7.7  d) •  Rocky  accretion  unlikely:  No   spectroscopic  metal  lines •  Most  likely  connected  to   pulsations Base of convection zone Surface Deeper
50. Most Modes Bounded by Base of Convection Zone •  Growth  time  ~days:     γ-­‐‑1  ≈  nτω         •  For  ~1000  s  modes: –  Radial  order  n  ~  20 –  τω   =   τthermal  at  top   of  mode  cavity   τω   ~  hrs Goldreich  &  Wu  I,  1999
51. Most Modes Bounded by Base of Convection Zone •  Growth  time  is  days  for  ~1000  s  (~1000  µμHz)  modes Full K2 light curve: PG1149+057
52. Potential Outburst Mechanisms in Cool DAVs •  Nonlinear  mode   coupling,  via   parametric   instability?   (Wu & Goldreich 1999) •  Still  an  open   question! The  two  outbursting   DAVs
53. Last Week We Discovered a Third Outbursting DAV
54. K2’s Expanding View of Pulsating White Dwarfs •  So  far,   20  pulsating  white  dwarfs  with   >1  month  short-­‐‑cadence  Kepler  data Not variable with K2 SC Not outbursting from LC First 3 DAVs that outburst Ground-based DAVs
55. §  Ensemble  Asteroseismology  of  White  Dwarfs §  White  Dwarf  Rotation  Rates §  Incidence  of  Magnetism  in  White  Dwarfs §  More  Remnant  Planetary  Systems §  Close,  Evolved  Binaries §  Refining  WDs  as  “Flux  Standards” What More Can We Expect from K2?
56. K2’s Expanding View of Pulsating White Dwarfs •  Fresh  data  from  K2  Campaign  5! •  New  11720(340)  K,  log(g)  =  8.20(0.11)  pulsating  WD Kp  =  17.1  mag
57. Christmas in July (and October, and March) Campaign  3 22  03  40.61  -­‐‑12  15  10.8 Kp=17.6  mag No  spectra,  high  proper  motion
58. 0.0 0.5 1.0 1.5 2.0 Phase 50 0 50 100 150 200 250 300 Radial Velocity (km/s) sssj2203 1.137 hr Porb  =  68.22  min K1  ~  69(8)  km/s   M2  >  0.07  Msun.     For  i=60,  M2  =  0.08  Msun Likely  a  WD+BD  binary very  close  to  Roche-­‐‑lobe  filling Christmas in July (and October, and March) 2.5m  INT  spectroscopy: 24,430(310)  K,  0.39(05)  Msun  WD
59. Campaign  4 Kp  =  17.7  mag 2.5m  INT  spectroscopy: 8840  K,  0.50  Msun  WD Christmas in July (and October, and March)
60. Φ  =  0.39 Φ  =  0.89
61. Mark Hollands, University of Warwick Rotation/observer  inclination:  45  ±  10  deg. Rotation/magnetism  colatitude:  38  ±  10  deg. Dipole  offset,  az  =  -­‐‑0.31  ±  0.04
62. White Dwarfs Aren’t All Great Flux Standards •  For  nonvariable  white   dwarfs,  stay  away  from   convective  surfaces:   DAs  <  13,000  K   DBs  <  25,000  K g-modes—remarkably similar to the large-amplitude DAV pulsators (Winget et al. 1 The observed pulsating white dwarf stars lie in three strips in the H-R diagram, in Figure 3. The pulsating pre-white dwarf PG 1159 stars, the DOVs, around 7 170,000 K have the highest number of detected modes. The first class of pulsating 5.5 5.0 4.5 Planetary Nebula Main sequence DOV DBV DAV 4.0 3.5 3.0 log [T eff (K)] 4 2 0 –2 –4 log (L/L ) Figure 3 A 13-Gyr isochrone with z = 0.019 from Marigo et al. (2007), on which we have drawn the ob Annu. Rev. Astro. Astrophys. 2008.46:157-199. Downloaded f by University of Texas - Austin on 01/28/09. For Winget & Kepler 2008
63. •  Confirmed  that  planets  are  disrupted  onto  white  dwarfs •  From  pulsations:  A  close  binary  WD  with  truncated  RGB   evolution  rotates  much  faster  than  isolated  WDs •  Outbursts  in  the  coolest  DAVs  with  very  deep  convection  zones What Have We Learned from K2 so Far?
64. §  Ensemble  Asteroseismology  of  White  Dwarfs §  Can  test  for  diversity  in  envelope  thicknesses,   which  would  affect  cooling  ages §  White  Dwarf  Rotation  Rates §  Incidence  of  Magnetism  in  White  Dwarfs §  More  Remnant  Planetary  Systems §  Close,  Evolved  Binaries §  Refining  WDs  as  “Flux  Standards” What More Can We Expect from K2?
65. K1: 20 WDs observed,