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The Cool Magnetic White Dwarf NLTT 10480*

22/07/2011

Unsurprisingly, most stars can be considered hot. However, physics being as it is, stars are often described as being “cool”, an ambiguous definition to say the least. To the uninitiated, a cool star could be considered as having an effective surface temperature of less than 5000K. To put this arbitrary value into context, out Sun has a mean averaged effective surface temperature of ~5770K. So, roughly speaking, the Sun is tepid. This figure is deliberately excluding the immense temperatures in the solar corona, which is usually quoted as one million Kelvin (Nakariakov et al. 2007).

Perhaps unbeknown to some readers, our Sun will end its life by shedding its outer layers. Expanding, quite devastatingly, into the wider solar system and entering the Red Giant (RG) phase. The inner core will eventually contract, forming a central nugget called a white dwarf (WD); perhaps as hot as 27000K. This is a typical effective temperature of a WD (Le Blanc 2010). It would therefore be quite rare to observe a WD that is considered “cool”. A rarity of sorts.

The interest in cool WDs lies in understanding their chemical composition, magnetic configuration, age and surface gravity, which seems to show a surprising amount of variety. Heavy elements, particularly calcium, are detected in close to a quarter of cool, hydrogen-rich (DA) WDs, but with abundances well below solar (Zuckerman et al. 2003). The abundance of heavy elements decreases with white dwarf cooling ages mainly because of the increasing depth of the mixed convective layers in aging WDs (Paquette et al. 1986; Koester 2009).

Few examples of very cool ( 5 000 K) polluted WDs (DAZ) are known, such as G 77-50 (WD 0322−019, Hintzen & Strittmatter 1974; Sion et al. 1990), which is also harbouring a weak magnetic field (Farihi et al. 2011). New high-dispersion and high signal to noise ratio spectroscopic observations of faint high propermotion stars are likely to contribute new objects to the current sample.

Kawka and Vennes (2011) have recently made some conclusive observations of what was previously an extreme rarity in our Galaxy. The rarity is known as “NLTT 10480*” (or LHS 5070, or LP 887-66 depending on which star catalogue is used). It is considered a high proper-motion star (Luyten 1979, 1980) that was also listed as a WD candidate in Luyten’s WD catalogue in 1977 (Luyten 1977).

Using an extensive array of spectral analysis techniques, and based on independent diagnostics (e.g. Ca I/Ca II ionization, as well as comparing the B-V colour index and the optical-to-infrared V − J colour index) Kawka and Vennes (2011) constrained an effective temperture for NLTT 10480* of 5200±200K. So, just on the cusp of being described as “very cool”. However, Kawka and Vennes (2011) noted systematic differences in temperature measurements based on the calcium ionization ratio, the colour index, and the Balmer line profiles amounting to ∼400 K. The weaker CaI lines favour a lower temperature than estimated using Balmer lines alone (the temperature measured with the V − J colour index also favours a lower temperature).

N.B. At this point I would rather point the reader towards: “Condon, E.U. & Shortley, G.H. (1963) The Theory Of Atomic Spectra. Cambridge University Press, Cambridge” as the vastness and the intricacies of atomic spectra are best suited to an undergraduate lecture series in atomic physics rather than here.

From this, as expected, NLTT 10480* also showed an abundance of heavier elements, as identified in its spectral arrangements. Heavy elements in the atmosphere of cool WDs are almost certainly accreted from their immediate environment (Kawkwa & Vennes 2011). Kilic et al. (2006) and Farihi et al. (2009) reported infrared observations of a sample of cool white dwarfs contaminated with heavy elements, and the authors noted an infrared-excess incidence of ≈10-20%.

This excess was attributed to debris discs of temperatures ranging from a few hundred degrees to over 1000 K. Some cool DAZ WDs, such as G 174−74 (WD 0245+541), do not show an infrared excess (Debes et al. 2007), and the presence of heavy elements in cool white dwarfs with ages in excess of several billion years suggests, instead, the effect of episodic accretion from small asteroids rather than from a stable debris disc (e.g. Jura 2008). Therefore, the identification of a new cool DAZ white dwarfs is of interest to constrain the phenomenon.

Journal References:

  • Sion, E.M.; Kenyon, S.J.; Aannestad, P.A. (1990) An Atlas Of Optical Spectra Of DZ White Dwarfs & Related ObjectsAstrophysical Journal Supplement Series, 72 pp.707-714.
  • Kawka, A & Vennes, S. (2006) Spectroscopic Identification Of Cool White Dwarfs In The Solar Neighborhood. The Astrophysical Journal, 642 (1) pp. 402-415.
  • Jura, M (2008) Pollution Of Single White Dwarfs By Accretion Of Many Small Asteroids. The Astrophysical Journal, 135 (5) pp. 1785-1792.
  • Kawka, A. & Vennes, S. (2011) The Cool Magnetic DAZ White Dwarf NLTT 10480*. Astronomy & Astrophysics, 532 Article I.D.: A7.
Suggested Preliminary Reading:
  • Condon, E.U. & Shortley, G.H. (1963) The Theory Of Atomic Spectra. Cambridge University Press,Cambridge

 

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