1 To Be or Not to Be: The Mysteries of Disk Formation Around Rapidly Spinning Be Stars Douglas R. Gies Department of Physics and Astronomy Center for High Angular Resolution Astronomy Georgia State University

2 Outline Introduction to the Be Stars Evolution of Interacting Binaries Be X-ray Binaries (Be + Neutron Star) CHARA Array Observations of Be Stars Ongoing and Future Work

3 Acknowledgements Current Students: Erika Grundstrom, Tabetha Boyajian, Steve Williams, Yamina Touhami, Noel Richardson, Ellyn Baines, Chris Farrington, Astr 8600 Past students: Ginny McSwain, Wenjin Huang, Reed Riddle, Dave Berger Colleagues: Hal McAlister, Theo ten Brummelaar, Bill Bagnuolo, David Wingert, Karen and Jon Bjorkman (Univ. Toledo)

4 Accretion and Angular Momentum Angular momentum = r x v In many gas accretion situations where r decreases with time, we find that the momentum ends up in a disk … Sun and planets: most of the mass in the Sun, but most of the angular momentum in the planets and Oort cloud

5

6 Disks around proto-stars

7 Disks around black holes

8 Disks around galactic nuclei

9 Too Much Angular Momentum: Be stars (massive stars with disks) B spectral type stars (11 – 30 kK) that are relatively unevolved (core H-burning) Circumstellar gas disks revealed by emission lines (hydrogen Balmer series), infrared excess continuum emission, and linear polarization (of scattered star light) Disk features inherently time variable: B Be B …(months to decades)

10 Detailed spectra show emission intensity is split into peaks to blue and red of line-center. o Intensity Wavelength Indicates a disk of gas orbits the star. This is from Doppler shift of gas moving toward and away from the observer. Hydrogen spectrum H H e = emission lines in the spectrum

11 Examples of Temporal Variations: Be stars in cluster NGC McSwain 2006

12 Gamma Cas

13 4 of 7 Sisters in Pleiades are Be stars

14 Be Stars are Rapid Rotators Spectral lines are broadened by rotation and the Doppler effect

15 How Close to Critical Rotation? Spectroscopy suggests Be stars rotate at 80% of critical rate (where centripetal acceleration = gravity at the equator) Townsend et al. (2004) show that gravity darkening will lead to an underestimate of the rotation rate 100% critical?

16 Temporal Variations: need rotation plus variable process Nonradial PulsationMagnetic Fields

17 Putting the Spin on Be Stars Why are Be stars rotating so quickly? born with high angular momentum experiencing a re-distribution of internal angular momentum near the conclusion of core hydrogen burning received mass and angular momentum through mass transfer from a binary companion (this must occur since spin-up observed in Algols and results of accretion seen in BeXRBs)

18 McSwain & Gies (2005) Be stars are neither very old nor very young Consistent with idea that many form in binaries

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20 Evolution of Interacting Binaries Many B-stars are members of close binary systems Stages: Be + He star φ Persei Be + neutron star Be X-ray binaries

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22 Going to the Extreme: BeXRBs SN results in neutron star in elliptical orbit Accretion X-ray flux should attain max. near periastron How large can disks grow in BeXRBs?

23 Grundstrom and Astr 8600 >3 year survey of three BeXRB systems Measured Hα strength in spectra from the KPNO Coudé Feed telescope Developed code for relationship between Hα strength and disk radius (dependent on disk temperature and inclination; Grundstrom & Gies 2006, ApJ, 651, L53) Documented disk radius and X-ray flux variations using NASA RXTE/All Sky Monitor instrument

24 LS I (P = 26.5 d, e = 0.55) (Grundstrom et al. 2007, ApJ, in press; astro-ph/ ) Be star + collapsed star with relativistic jets, gamma ray emission (microquasar) Orbit: e = 0.55 Mean disk radius R d / R s 4.6 (4:1 resonance) Historical max. R d / R s 5.6 ( periastron) Photoionization of disk in 1 day?

25 HDE = A (P = 110 d, e = 0.47) (Grundstrom et al. 2007, ApJ, submitted) No disk in 1998 Recent disk radius R d / R s 5 (5:1 resonance?) Historical max. R d / R s 9 ( periastron)

26 X Persei (P = 250 d, e = 0.11) (Grundstrom et al. 2007, ApJ, submitted) Disk growth to record strength Current disk radius R d / R s 6.4 But component separation is large (Roche radius at periastron = 34 R s ) how does gas cross the gap to NS?

27 Feeding the X-ray Source All three show that X-ray max. occurs P/4 after periastron Suggests disk becomes extended by tidal forces at periastron LS I

28 Okazaki et al. (2002)

29 I Can See Clearly Now: Direct Resolution with the CHARA Array Hα disks observed by Tycner et al. Expect IR excess from ionized gas f-f and b-f emission Should appear in K-band (λ = 2.1μm) Waters et al. (1991)

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31 CHARA Array Observations (Gies et al. 2007, ApJ, 654, Jan. 1; astro-ph/ ) K-band interferometric observations of four classical Be stars (2003 – 2005) Moderate to long baselines CHARA Classic beam combiner Observations interposed with calibrator stars with known angular diameter in order to transform instrumental fringe visibility into absolute visibility V V = Fourier transform of angular image

32 Models of K-band Visibility Uniform disk star with set angular diameter (π, R s ) Disk geometry (Hummel & Vrancken 2000) ρ(R,Z) = ρ 0 R -n exp[-0.5(Z/H(R)) 2 ] ρ 0 = base density (g cm -3 ) n = radial density exponent H(R) = R 3/2 C s / V K = disk scale height Observer parameters i = inclination of disk normal α= position angle (E from N) of disk normal

33 Models of K-band Visibility Isothermal disk T d = 0.6 T eff (star) (Carciofi & Bjorkman 2006) maximum emission: Planck function for T d IR free-free and bound-free optical depth (Waters 1986; Dougherty et al. 1994) IDL code: integrates ρ 2 along rays through disk I = S d (1-e -τ ) + S * e -τ S d = source function for disk S * = source function for uniform star Fourier transform images to get visibility V (Aufdenberg et al. 2006)

34 γ Cas: i=51º, ρ 0 =7.2x10 -11, n=2.7 Minor axis Major axis

35 γ Cas: i=80º, ρ 0 =7.2x10 -11, n=2.7 …. = original model with i=51º, ρ 0 =7.2x10 -11, n=2.7

36 γ Cas: i=51º, ρ 0 =3.6x10 -11, n=2.7 …. = original model with i=51º, ρ 0 =7.2x10 -11, n=2.7

37 γ Cas: i=51º, ρ 0 =7.2x10 -11, n=2.0 …. = original model with i=51º, ρ 0 =7.2x10 -11, n=2.7

38 Fitting the Models Search for χ 2 minimum for ρ 0, n, i, α All four targets are known binaries, but nature of companion unknown for all but the case of φ Per Determined both fits as single Be and as Be plus hot subdwarf companion inclusion of companion significantly improved fits for κ Dra and φ Per

39 γ Cas: single star fit α=116º, i=51º, ρ 0 =7x10 -11, n=2.7

40 φ Per: binary with P = d α=49º, i=69º, ρ 0 =1x10 -11, n=1.8

41 ζ Tau: single star fit α=38º, i=90º, ρ 0 =2x10 -10, n=3.1

42 κ Dra: binary fit with P = 61.6 d α=21º, i=26º, ρ 0 =6x10 -13, n=0.7

43 Sanity Checks: IR Excess Parameterγ Casφ Perζ Tauκ Dra E(V-K) (Dougherty et al.) E(V-K) (K model) Disk densities may have varied over 15 years between the IR and CHARA Array measurements

44 Sanity Checks: Hα Interferometry Parameterγ Casφ Perζ Tauκ Dra α (MkIII) … α (NPOI) … α (CHARA) i (MkIII)4663>74… i (NPOI)55>55>74… i (CHARA) θ (MkIII) … θ (NPOI) … θ (CHARA)

45 Summary BeXRBs: ideal setting to follow disk growth and accretion fueled X-ray variations Nearby Be disks can be resolved with the CHARA Array Disks are smaller in K-band than in Hα Total disk mass ranges from 8x10 -8 (κ Dra) to 2x10 -6 (γ Cas) solar masses Disk filling time 1 year (BeXRBs)

46 Summary If we assume (1) mass loss occurs at the stellar equator and (2) disk gas never returns, then we can estimate the rate of angular momentum transferred into the disk: dJ/dt = -dM/dt V eq R s Time scale for spin down is J / dJ/dt ¼ main sequence lifetime This suggests that disk formation is the solution of the angular momentum problem for Be stars

47 A Future So Bright: Work Underway Grundstrom dissertation: KPNO Coude Feed Telescope survey of 130 Be stars

48 Be Spectral Energy Distribution NASA IRTF: Flux excess in K, L bands to constrain F disk / F star in models for CHARA Array interferometry

49 Upcoming CHARA Program Ellyn Baines observed Be stars 59 Cyg and υ Cyg this past summer Yamina Touhami will observe γ Cas next week with FLUOR (better S/N and bigger disk) Yamina will use CHARA Classic in the following week for a quick survey of Be stars just observed from KPNO Coude Feed: six targets should have K-band disk diameters larger than 1.7 mas FWHM (based upon Hα strength)

50 Upcoming CHARA Program ο Cas W λ = -32 Å Predicted K-band diameter is 3.7 mas FWHM largest yet

51 Long Range Plans with CHARA Structure in Be disks – spiral arms Time evolution of disks – follow expansion and dissipation Find elusive companions – source of Be spin

52 Hamlet's Soliloquy Revised (or what if Hamlet had taken up astrophysics) HAMLET: To be, or not to be - that is the question: GIES: To Be, or not to Be - that is the question:

53 Hamlet's Soliloquy Revised (with apologies to Shakespeare) HAMLET: Whether 'tis nobler in the mind to suffer GIES: Whether disks overflow in time and suffer

54 Hamlet's Soliloquy Revised (with apologies to Shakespeare) HAMLET: The slings and arrows of outrageous fortune GIES: The peaks and troughs of outrageous pulsation

55 Hamlet's Soliloquy Revised (with apologies to Shakespeare) HAMLET: Or to take arms against a sea of troubles GIES: Or to make harm against a B [field] of troubles

56 Hamlet's Soliloquy Revised (with apologies to Shakespeare) HAMLET: And by opposing end them. To die, to sleep - GIES: And by ejection end them. To try, to keep -

57 Hamlet's Soliloquy Revised (with apologies to Shakespeare) HAMLET: No more - and by a sleep to say we end GIES: fringes galore - and by good scans today we end

58 Hamlet's Soliloquy Revised (with apologies to Shakespeare) HAMLET: The heartache, and the thousand natural shocks GIES: The heartache, and the thousand perverse knocks

59 Hamlet's Soliloquy Revised (with apologies to Shakespeare) HAMLET: That flesh is heir to. 'Tis a consummation GIES: That interferometry is heir to. 'Tis an observation

60 Hamlet's Soliloquy Revised (with apologies to Shakespeare) HAMLET: Devoutly to be wished. To die, to sleep - GIES: Devoutly to be wished. To try, to keep -

61 Hamlet's Soliloquy Revised (with apologies to Shakespeare) HAMLET: To sleep - perchance to dream: ay, there's the rub, GIES: To model - develop a scheme; ay, there's the rub,

62 Hamlet's Soliloquy Revised (with apologies to Shakespeare) HAMLET: For in that sleep of death what dreams may come GIES: For in that chi-squared fit what bugs may come

63 Hamlet's Soliloquy Revised (with apologies to Shakespeare) HAMLET: When we have shuffled off this mortal coil, GIES: When data reduction takes its mortal toil

64 Hamlet's Soliloquy Revised (with apologies to Shakespeare) HAMLET: Must give us pause. There's the respect GIES: Must give us pause. There's the aspect

65 Hamlet's Soliloquy Revised (with apologies to Shakespeare) HAMLET: That makes calamity of so long life … GIES: That takes ones sanity to the brink in life …