IRAS 08544−4431

IRAS 08544−4431 is a binary system surrounded by a dusty ring in the constellation of Vela. The system contains an RV Tauri variable star and a more massive but much less luminous companion.

IRAS 08544−4431

Warm circumstellar material around IRAS 08544-4431
Credit: ESO
Observation data
Epoch J2000.0      Equinox J2000.0
Constellation Vela
Right ascension  08h 56m 14.181s[1]
Declination −44° 43′ 10.71″[1]
Apparent magnitude (V) 9.01 - 9.19[2]
Characteristics
Spectral type F3e[3]
Variable type RV[2]
Astrometry
Radial velocity (Rv)+62.5[3] km/s
Proper motion (μ) RA: −7.8[1] mas/yr
Dec.: +9.5[1] mas/yr
Parallax (π)0.6551 ± 0.0767[4] mas
Distanceapprox. 5,000 ly
(approx. 1,500 pc)
Orbit[5]
Period (P)506.0±1.3 d
Eccentricity (e)0.22±0.02
Inclination (i)20±7.5°
Argument of periastron (ω)
(secondary)		41.7±6.3°
Semi-amplitude (K1)
(primary)		87±0.2 km/s
Details
A
Mass0.65[6] M☉
Radius75[5] R☉
Luminosity14,000[5] L☉
Surface gravity (log g)1.5[7] cgs
Temperature7,250[7] K
Metallicity [Fe/H]−0.5[3] dex
B
Mass1.15[6] M☉
Temperature4,000[8] K
Other designations
V390 Velorum, CD−44 4961, IRAS 08544−4431, 2MASS J08561419−4443107
Database references
SIMBADdata

Binary
The dusty ring around IRAS 08544−4431

In 2003, IRAS 08544−4431 was being studied as a likely RV Tauri variable and was identified as a binary star from periodic variations in its observed radial velocity. The primary is a luminous F3 star surrounded by a dusty disc, and the invisible secondary is a less massive star.

The two components of IRAS 08544−4431 orbit in 499 days in a mildly eccentric orbit. The projected semi-major axis is 0.32 AU but the inclination of the orbit is not known so the actual separation may be considerably larger, although the inclination is thought to be fairly large because the type of brightness variation implies a face-on system.[3]

Variability

IRAS 08544-4431 is classified as an RV Tauri star, a type of pulsating variable star which shows cycles with alternating shallow and deep minima. In addition, IRAS 08544-4431 shows slow variations in amplitude from cycle to cycle over approximately 1,600 days, a defining characteristic of a type b RV Tauri variable. The maximum amplitude is only 0.18 magnitudes.[9] It was given the variable star designation of V390 Velorum in 2006.[10]

The period, defined for an RV Tau star as the time between two deep minima, is 72 days. The slow variations in amplitude have been measured, represented by a period of 69 days producing beats. None of these variations correspond to the orbital motion.[9]

Post-AGB

The primary star is thought to be a post-AGB star, a highly evolved star that has ceased fusion and is ejecting its outer layers on its way to becoming a white dwarf. Although many post-AGB stars become planetary nebulae once they become hot enough to ionise their ejected outer layers, it is thought that IRAS 08544−4431 is not massive enough to do this.[7]

Dusty disc
The disc around IRAS 08544−4431 at 2μm and 10μm. Although the image resembles a doughnut lit from the front, the brightest arc is the inner wall of the far side of the disc, lit by the stars at the centre.

The warm material surrounding IRAS 08544−4431 has been resolved using interferometry with the AMBER and MIDI instruments at the Very Large Telescope. It is a circumbinary disc surrounding both stars, is heated mainly by the primary post-AGB star, and has a total mass of 0.015 M☉. The disc starts 9 AU from the stars and is approximately 4 AU thick at its inner edge. The thick disc protects much of the dust from direct heating out to 70 AU from the stars. Beyond 70 AU, the disc is thick enough to receive direct radiation from the stars.[11]

The disc is at a temperature of 1,150 K. Although the companion is far less luminous than the primary, it is brighter than expected, especially at infrared wavelengths. It is suspected to be a main sequence star with its own compact accretion disc. The best images of the disc and stars, taken using the PIONIER interferometer, show the primary star to be 0.5 mas across, the secondary to be an unresolved point source 0.91 mas away, and the circumbinary disc to be 14.15 mas in diameter. The disc is oriented at 19° to the plane of the sky aligned at an angle of about 6° away from N-S.[8]

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References
  1. Høg, E.; Fabricius, C.; Makarov, V. V.; Urban, S.; Corbin, T.; Wycoff, G.; Bastian, U.; Schwekendiek, P.; Wicenec, A. (2000). "The Tycho-2 catalogue of the 2.5 million brightest stars". Astronomy and Astrophysics. 355: L27. Bibcode:2000A&A...355L..27H.
  2. Samus, N. N.; Durlevich, O. V.; et al. (2009). "VizieR Online Data Catalog: General Catalogue of Variable Stars (Samus+ 2007-2013)". VizieR On-line Data Catalog: B/gcvs. Originally Published in: 2009yCat....102025S. 1. Bibcode:2009yCat....102025S.
  3. Maas, T.; Van Winckel, H.; Lloyd Evans, T.; Nyman, L.-Å.; Kilkenny, D.; Martinez, P.; Marang, F.; Van Wyk, F. (2003). "IRAS 08544-4431: A new post-AGB star in a binary system surrounded by a dusty disc". Astronomy and Astrophysics. 405: 271–283. arXiv:astro-ph/0306492. Bibcode:2003A&A...405..271M. doi:10.1051/0004-6361:20030613.
  4. Brown, A. G. A.; et al. (Gaia collaboration) (August 2018). "Gaia Data Release 2: Summary of the contents and survey properties". Astronomy & Astrophysics. 616. A1. arXiv:1804.09365. Bibcode:2018A&A...616A...1G. doi:10.1051/0004-6361/201833051. Gaia DR2 record for this source at VizieR.
  5. Kluska, J.; Hillen, M.; Van Winckel, H.; Manick, R.; Min, M.; Regibo, S.; Royer, P.; van Wyk, F. (2018). "The perturbed sublimation rim of the dust disk around the post-AGB binary IRAS08544-4431". Astronomy and Astrophysics. 616: A153. arXiv:1805.04501. Bibcode:2018A&A...616A.153K. doi:10.1051/0004-6361/201832983.
  6. Bujarrabal, V.; Castro-Carrizo, A.; Winckel, H. Van; Alcolea, J.; Contreras, C. Sánchez; Santander-García, M.; Hillen, M.; van Wyk, F. (2018). "High-resolution observations of IRAS 08544-4431. Detection of a disk orbiting a post-AGB star and of a slow disk wind". Astronomy and Astrophysics. 614: A58. arXiv:1802.04019. Bibcode:2018A&A...614A..58B. doi:10.1051/0004-6361/201732422. PMC 6031302. PMID 29983449.
  7. Stasińska, G.; Szczerba, R.; Schmidt, M.; Siódmiak, N. (2006). "Post-AGB stars as testbeds of nucleosynthesis in AGB stars". Astronomy and Astrophysics. 450 (2): 701. arXiv:astro-ph/0601504. Bibcode:2006A&A...450..701S. doi:10.1051/0004-6361:20053553.
  8. Hillen, M.; Kluska, J.; Le Bouquin, J.-B.; Van Winckel, H.; Berger, J.-P.; Kamath, D.; Bujarrabal, V. (2016). "Imaging the dust sublimation front of a circumbinary disk". Astronomy & Astrophysics. 588: L1. arXiv:1603.03023. Bibcode:2016A&A...588L...1H. doi:10.1051/0004-6361/201628125.
  9. Kiss, L. L.; Derekas, A.; Szabó, Gy. M.; Bedding, T. R.; Szabados, L. (2007). "Defining the instability strip of pulsating post-AGB binary stars from ASAS and NSVS photometry". Monthly Notices of the Royal Astronomical Society. 375 (4): 1338–1348. arXiv:astro-ph/0612217. Bibcode:2007MNRAS.375.1338K. doi:10.1111/j.1365-2966.2006.11387.x.
  10. Kazarovets, E. V.; Samus, N. N.; Durlevich, O. V.; Kireeva, N. N.; Pastukhova, E. N. (2006). "The 78th Name-List of Variable Stars". Information Bulletin on Variable Stars. 5721: 1. Bibcode:2006IBVS.5721....1K.
  11. Deroo, P.; Acke, B.; Verhoelst, T.; Dominik, C.; Tatulli, E.; Van Winckel, H. (2007). "AMBER and MIDI interferometric observations of the post-AGB binary IRAS 08544-4431: The circumbinary disc resolved" (PDF). Astronomy & Astrophysics. 474 (3): L45. Bibcode:2007A&A...474L..45D. doi:10.1051/0004-6361:20078079.
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