Hot Stars as Targets for Intensity Interferometry

Hannes Jensen Lund Observatory, Sweden Aims

• Given recent progress in phase interferometry: which should be the prime observational targets for II? • What is least difficult to observe? • Starting point for observing programs, white papers, etc. Selection Criteria

What makes a target suitable for II? • Bright High above background noise level, moonlight etc. • Hot S/N improves greatly with target temperature • Astronomically interesting Features on a (sub)milliarcsecond level. Deformation due to rapid rotation, pulsation, disks, winds, etc. The Bright Star Catalogue

The Bright Star Catalogue contains the ~9000 brightest stars. Temperatures

Effective temperatures were approximated with a polynomial fit to values from [Bessel et al, 1998, A&A333].

~2600 objects are hotter than 9000 K and brighter than V=7 Final selection

• 34 stars exist with (Teff > 9000 K ∧ V < 2) ∨ T eff > 25000 K • A few other interesting objects were added • Angular diameters from the CHARM2 compilation. [Richichi et al., 2005, A&A431, 773] Selected objects

Temperature vs. apparent magnitude of selected stars. Circle size indicates angular diameter where available.

1 mas List of Objects

Angular Rot. vel.* Name Spec. class Teff [K] V [mag] Notes size (mas) [km/s]

Achernar, HR 472 1.9 250 B3Ve 15000 0.46 High rotational velocity, Be-star

Rigel, HR 1713 2.4 30 B8Iab 9800 0.12 Emission line star, SN candidate

Lambda Lep, HR 1756 70 B0.5IV 28000 4.29

Bellatrix, HR 1790 0.7 60 B2III 21000 1.64 Variable star

Elnath, HR 1791 1.5 70 B7III 13000 1.65 Star in double system

HR 1855 20 B0V 28000 4.62 Variable star

HR 1887 38 B0.5V 26000 4.78 Star in double system

Alnilam A, HR 1903 0.7 87 B0Iab 18000 1.7 Emission line star

HR 1996 153 O9.5V 33000 5.17 High rotational velocity

Beta CMa 0.5 36 B1II/III 23000 1.98 Short period beta Cep variable

Alhena, HR 2421 1.4 30 A0IV 9100 1.93 Star in double system

S Mon, HR 2456 60 O7Ve 26000 4.66

Sirius, HR 2491 5.9 13 A1V 9100 -1.46 Bright, well studied, high angular size

EZ CMa, HR 2583 ? WN4 33000 6.91 Highly variable W-R star

Adara, HR 2618 0.8 44 B2Iab 20000 1.5 Star in double system

Naos, HR 3165 0.4 211 O5Ia 28000 2.25 BY Dra variable, high rot. vel.

HR 3207 0.4 WCv 21000 1.78 Double or multiple star

HR 3685 1.5 133 A2IV 9100 1.68

* vrot sin i List of Objects, contd.

Angular Rot. vel. [km/ Name Spec. class Teff [K] V [mag] Notes size (mas) s]

Beta Cen, HR 5267 0.9 264 B1III 23000 0.61 Beta Cep variable

Tau Sco, HR 6165 24 B0.2V 26000 2.82

Lambda Sco, HR 6527 163 B2IV 21000 1.63 Beta Cep variable

Kaus Austr., HR 6879 1.4 147 B9.5III 9800 1.85 Star in double system

Vega, HR 7001 3.2 ? A0V 9100 0.03 Bright, well studied

Peacock, HR 7790 0.8 39 B2IV 19000 1.94 Spectroscopic binary

Deneb, HR 7924 2.2 21 A2Iae 9300 1.25 Alpha Cyg variable

HR 8425 1.0 236 B6V 13000 1.74 High rotational velocity

Fomalhaut, HR 8728 2 100 A4V 9300 1.16 First optically imaged exo-planet

Eta Car, HR 4210 5.0 (CSS) ? pec 36000 6.21 Highly unstable, SN candidate?

WD 0005+511 ? 200 000 13.23 Hottest WD known

Gamma Vel, HR 3207 0.4 WC8+O9I 1.8 WR-O binary

Regulus, HR 3982 1.4 329 B7V 12000 1.35 High rotational velocity

Acrux, HR 4730 117 B0.5IV 24000 1.33 Spectroscopic binary

HR 4731 197 B1V 28000 1.73 High rotational velocity

Beta Cru, HR 4853 0.7 38 B0.5IV 23000 1.25 Beta Cep variable

Eps UMa, HR 4905 38 A0p 9500 1.77 Variable star

Spica, HR 5056 0.9 159 B1III-IV 23000 0.98 High rotational velocity, variable

Alcaid, HR 5191 < 2. 205 B3V 18000 1.86 High rotational velocity, variable Selected objects Achernar (B3Ve)

Teff 15 000 K • Very rapidly rotating Be-star V [mag] 0.46 v sin i 250 km/s • Highly deformed due to rotation, observed with VLTI θ 1.85 mas [de Souza et al., 2003, A&A407, 47] • Complex stellar winds due to rotation [e.g., Kervella et al., 2008] • Circumstellar disk may be studied with II in its Hα emission • Companion star suggested [Kervella & de Souza, 2007, A&A474, 49]

[Kanaan et al., 2008, A&A, 486] Rigel (B8Iab)

Teff 10 000 K The nearest blue supergiant V [mag] 0.12 • v sin i 30 km/s θ 2.4 mas • Nearest type II supernova progenitor? [Stewart et al., 2009] • Periodic oscillations on many timescales [Stewart et al., 2009] • Variation in absorption/emission in Hα lines [Morrison et al., 2009] β Centauri (B1III)

Teff 22 000 K* V [mag] 0.61 • The brightest β Cep variable v sin i 140 km/s θ 0.9 mas

• Oscillations, line profile variations *both components approx. equal mass and temp. • Highly complex spectroscopic binary, both components variable [Ausseloos et al., 2006, A&A455] • “A challenge for current evolution scenarios in close binaries” [Ausseloos et al., 2002 A&A209] • Orbits measured with Sydney University Stellar Interferometer (SUSI) [Davis et al, 2005,MNRAS 256 1362] Vega (A0 V)

Teff 9 000 K Widely studied standard star. “Arguably the second most V [mag] 0.03 • v sin i 15 km/s important star in the sky after the Sun” [Gulliver et al., 94, ApJ429] θ 3.2 mas • Realized to be a rapidly rotating pole-on star • Center-limb intensities: 18x drop at 500 nm, compared to 5x drop for non-rotating model [Peterson et al., 2006, Nature 440, 896] Fomalhaut (A4V)

Teff 9 000 K Host star of an optically imaged exoplanet V [mag] 1.16 • v sin i 100 km/s θ 2 mas • Debris disk with planet(s) η Carinae

Teff 36 000 K Most luminous star known in the Galaxy V [mag] 6.21 • v sin i ? θ 5 mas* • Highly unstable, “doomed to explode within 100 000 years” * Circumstellar shell • Imaged with HST and VLT to some 0.1 arcsec; • VLTI data to milliarcsecond level shows asymmetries in winds due to rapid rotation [Weigelt et al., 2007, A&A, 464, 87]

Images: eso.org γ2 Velorum (WC8+O9I)

Teff 35 000 K Closest and brightest Wolf-Rayet star. V [mag] 1.8 • v sin i ? θ 0.4 mas • WR-O binary, important for understanding stellar evolution [North et al., 2007, MNRAS 377] • Excellent system for studying massive stars and their interacting winds [Millour et al., 2007, A&A464]

• Already observed at Narrabri by Hanbury Brown et al.: Angular size in C III-IV emission at 465 nm shows stellar envelope five times larger than stellar disk in the continuum Conclusions

• Observationally suitable and exciting targets exist all over the sky • Much to be gained from sub-milliarcsecond measurements - way more than just stellar diameters!