The science potential of atmospheric Cherenkov arrays used as intensity interferometers

Transcription

1 The science potential of atmospheric Cherenkov arrays used as intensity interferometers Michael Daniel for Willem-Jan de Wit

2 Atmospheric Cherenkov Telescope Arrays Multiple telescopes image the source of optical Cherenkov light created by charged secondary particles ( the shower ) from an incoming gamma-ray. The faint Cherenkov light requires large light collectors. The brief (~nanosecond) Cherenkov flash requires fast photon detectors. Shower image reconstruction gives spectral and angular information on the incoming gamma-ray, hence many telescopes covering long baselines. H.E.S.S. VERITAS

3 Similar technical specs of ACTs and I.I. A proposed CTA lay-out Fast photon counters/digital processing (very) Large photon collectors Large number of telescopes Telescope can point independently Long baselines red dots : 85m2 dish blue dots : 600 m2 dish 89 telescopes = 3916 baselines e.g. Le Bohec & Holder (2006) ACT projects under study: CTA (EU) AGIS (US) Bernloehr et al. (2007)

4 What do stellar astronomers want? (apart from some fun) The accurate determination of the physical properties of stars, i.e. mass, radius, luminosity and elemental abundances... and everything that either is ex- or accreted from/onto the star.

5 Sensitivity of CTA as an I.I. S/N = 5 A = m2 quantum efficiency 30% n = 5.0 e-5 (1.0e-4) degree of coherence = 0.5 bandwidth 1 GHz exposure time 5h 2 S / N RMS= A n ph d f T /2 (from Hanbury Brown 1973) Le Bohec & Holder (2006) limiting magnitude mv=8.5 [mv= 9.25 using flux zeropoint of Bessel et al (1990)]

6 In a simple perspective... How many main sequence stars can be imaged (Mv<9.25)? Three science case examples: Young stars Rapidly rotating stars Cepheids and distance scale

7 Science case: young stars the internal stellar structure of pre-main sequence stars chromospheric activity (cool spots) accretion activity (hot spots) Nearby star formation regions: Palla & Stahler (2000)

8 Science case: young stars In the solar neighbourhood ~50 young stars with mv<8m In the last decade several young coeval stellar groups have been discovered in close proximity (~50pc) to the sun. Their closeness means the members are bright and renders the co-moving group relatively sparse making them suitable, unconfused, targets even with the large optical PSF for an IACT (~few arcminutes). The spectral type range from A to G Sparse, therefore incomplete membership (N(*) will increase!) Ages between 8-50Myr: a substantial fraction still in the pre-main sequence contraction phase Zuckerman & Song (2004) 50pc oval: 50 pc small blue dots: Beta Pic Ass. small red dots: Tucana/Horologium Ass. grey dot: Pleiades large blue dot: Beta Pic

9 The internal structure of young stars and the calibration of pre-main sequence tracks... From PMS tracks masses are inferred. They are therefore fundamental for a correct understanding of the star formation process. Interferometry of (non-eclipsing) spectroscopic binaries deliver dynamical masses (e.g. Boden et al. 2005), in combination with spectroscopic data. Components V mags: 6.9 and 8.0 at 45pc Based on 34 Keck-I interferometry measurements In addition: Known distances (Hipparcos, GAIA [launch 2011]) allow direct comparison of the predicted and I.I. observed sizes of individual PMS stars. Boden et al. 2005

10 The surface structure of young stars Understanding chromospheric activity and stellar magnetic fields cool spots (similar to sunspots) may cover 50% of the stellar surface and are the product of the slowly decaying rapid rotation of young stars hot spots deliver direct information regarding the accretion of material onto the stellar surface T Tauri Magnetically guided accretion process (accretion funnel)

11 The surface structure of young stars Understanding chromospheric activity and stellar magnetic fields Inferred brightness distribution from Doppler imaging Doppler images of PW And 20Myr, K2V T Tauri 40pc Magnitude in V-Band 8.7 Cool spots are 1200K cooler than stellar photosphere Cool spots may cover ~50% of surface > 65 stars have been doppler imaged (Strassmeier 2002) Strassmeier & Rice (2006)

12 The surface structure of young stars Hot spots and the magnetic accretion phenomenon 2Myr, M0 T Tauri 150pc Magnitude in V-Band 14 Accretion spots are 2000K hotter than stellar photosphere Doppler imaging of MN Lupi =>

13 The surface structure of young stars Hot spots and the magnetic accretion phenomenon inferred morphology in agreement with the magnetospheric accretion model the star's fast rotation (a factor of ~4 from break-up) suggests that accretion could be responsible for spin-up, and hence strong activity magnetic field strength can be inferred from comparison with models (Strassmeier et al. 2005)

14 Science case: distance scale The zero-point of the Cepheid period-luminosity relation Angular size estimate from I.I. and the Cepheid radius estimate from Baade-Wesselink method Baade-Wesselink method: 1. (vspectro* t) = R2-R1 2. L1/L2 = R1/R2 R2 R1 T1 T2 The radius of the cepheid can be determined from the observed radial velocities. dr =v t dt high resolution spectroscopy

15 Science case: distance scale The sizes of Cepheids Angular size measurement (θ) from intensity interferometry D θ Comparison of angular size and Baade-Wesselink determined size gives the distance to the Cepheid Davis et al using Sydney University Stellar interferometer (1 Cepheid) Lane et al. 2000/2002 with the Palomar testbed interferometer (2 Cepheids) Kervella et al with the Very Large Telescope Interferometer (7 Cepheids) ~60 cepheid variables with mv<8m +

16 Science case: rapidly rotating stars Extremely distorted stars near (at?) break-up velocity Classical Be stars are well-known for the presence of a circumstellar gaseous disk (the e in Be-type stars). The disk is formed in a mass-loss process from the star, and comes and goes (timescale of months to decades). It is bright in the NIR (Brehmsstrahlung). ζ Tauri in Hα at MkIII optical interferometer (2 telescopes) Quirrenbach et al. (1994)

17 Science case: rapidly rotating stars Classical Be disks : how to make them? Be stars are extremely fast rotators How close to break-up? Insensitivity of line-width to rotational velocity Pulsations probably also an important ingredient Townsend et al Von Zeipel effect/gravity darkening 0km/s 300km/s 400km/s 487km/s spectral measurements of rotation can only provide lower limits owocki

18 Science case: rapidly rotating stars The measurement of a deformed star VLTI-VINCI (K-band!, 2 telescopes) alpha Eridani: «Flattest star ever seen» v_rot (spectro) ~225 km/s v_rot (interfero) ~350 km/s (~break-up) Domiciano de Souza et al. 2003

19 Science case: rapidly rotating stars Extremely distorted stars near (at?) break-up velocity ~300 stars with mv<8m corresponding to a distance limit of 700pc 50% fraction of B0 stars showing Be phenomenon (i.e. an important concept within stellar evolutionary theory) The disk formation and dissolution are poorly understood

20 Simple examples: baseline coverage 89 telescopes Nbaseline = Ntel * (Ntel-1)/2. = 3916 (1060 independent baselines)

21 Simple examples I : perfectly dark spot 3 milli arcsecond 1 single observation with a full 89 telescope CTA array, using phase information. Original Image Power spectrum Reconstructed image

22 Simple examples II : Be star 3 milli arcsecond Original Image 1 single observation with a full 89 telescope CTA array, using phase information. Power spectrum Reconstructed image

23 Simple examples III : T Tauri 3 milli arcsecond Original Image 1 single observation with a full 89 telescope CTA array, using phase information. Power spectrum Reconstructed image

24 Science Summary Young stars: - Internal stellar structure by means of dynamical masses of binaries - PMS stellar radii in combination with known distances (GAIA) - Stellar rotation, cool spots and dynamo action - Hot spots and accretion phenomena (less certain) At least 50 young stars for which CTA-I.I. could provide images Distance scale: - Sizes of Cepheid variables to calibrate zeropoint of period-luminosity relation Rapidly rotating stars: - Unambiguous determination of the rotational velocities of Be stars Not or briefly discussed: X-ray binaries : 15 HMXB and 2 LMXB are brighter than 9.25 in V-band (from the on-line X-ray binary catalogue ( Planetary transits (perfectly dark spot) Since astronomers study objects beyond their control, an imaging CTA-I.I. will provide a wealth of new discoveries

25 Further reading The potential for intensity interferometry with γ-ray telescope arrays de Wit et al. arxiv: Towards μ-arcsecond spatial resolution with Air Cherenkov Telescope arrays as optical intensity interferometers de Wit et al. arxiv:

26 fin?