Altair – the “hottest” magnetically active star in X-rays

Abstract

Context. The onset of stellar magnetic activity is related to the operation of dynamo processes that require the development of an outer convective layer. This transition of stellar interior structure is expected to occur in late A-type stars. Aims. The A7 star Altair is one of the hottest magnetically active stars. Its proximity to the Sun allows a detailed investigation of a corona in X-rays for a star with a shallow convection zone. Methods. We used a deep XMM-Newton observation of Altair and analyzed X-ray light curves, spectra, and emission lines. We investigated the temporal behavior and properties of the X-ray emitting plasma and studied the global coronal structure of Altair. Results. Altair's corona with an X-ray luminosity of L X = 1.4 × 10 27 erg/s and an activity level of log L X /L bol = −7.4 is located predominantly at low latitude regions and exhibits X-ray properties that are overall very similar to those of other weakly active stars. The X-ray emission is dominated by cool plasma (1-4 MK) at low density, and elemental abundances exhibit a solar-like FIP effect and Ne/O ratio. The X-ray brightness varies by 30% over the observation, most likely due to rotational modulation and minor activity; in contrast, no strong flares or significant amounts of hot plasma were detected. The X-ray activity level of Altair is apparently close to the saturation level, which is reduced by roughly four orders of magnitude when compared to late-type stars. Conclusions. With its fast rotation, Altair provides an inefficient, but very stable dynamo that mainly operates in convective layers below its "cooler" surface areas around the equator. This dynamo mechanism results in magnetic activity and leads to X-ray properties that are similar to those of the quiescent Sun, despite very different underlying stars.

Magnetism, dynamo action and the solar-stellar connection

“…Stellar coronae and transition regions fade away in this mass range, as probed by UV and X-ray observations (see, e.g., Vaiana et al 1981;Pallavicini et al 1981;Schmitt et al 1985;Rosner et al 1985;Güdel 2004;Robrade and Schmitt 2009). The interpretation is that at spectral types between roughly B8 and A7, there is not enough non-radiative heating to heat the atmosphere to the temperatures required for such emission; such heating in solar-type stars is linked to surface convection and magnetism, so its absence is consistent with the disappearance of prominent near-surface convection zones.…”

“…But this is in the context of a overall stellar radii of order 10-20 R .) As the convective envelope shrinks in extent, though, a convective core develops: by the time surface convection zones have nearly disappeared in the mid-A stars (e.g., Robrade and Schmitt 2009), the convective core occupies the inner ∼15% of the star by radius. These changes in structure are a consequence of changes in the nuclear energy generation and opacity of the material at varying temperature and density (as discussed in Sect.…”

X-ray emission from the remarkable A-type star HR 8799

“…These stars have X-ray luminosities of one to a few times 10 27 erg/s and activity levels of log L X /L bol −7. Compared to Altair, the hottest magnetically active star studied in detail at X-ray energies (Robrade & Schmitt 2009), the X-ray surface flux of HR 8799 is higher by about a factor of 20; further the average coronal temperature of HR 8799 is with 3.0 MK slightly higher than that of Altair which is about 2.5 MK, but within the errors this difference is not very significant. This matches the finding that Altair's activity level is at log L X /L bol = −7.4 very low, even when compared to the weakly active stars HR 8799.…”

“…Schmitt et al 1985;Schmitt 1997). An XMM-Newton observation of the fast rotating A7 star Altair confirmed the presence of weak magnetic activity and coronal X-ray emission (Robrade & Schmitt 2009). The vanishing of magnetic activity in mid A-type stars is expected theoretically and confirmed by other activity indicators; for instance FUSE observations of main sequence stars have shown the disappearance of chromospheric emission lines at effective temperatures above T eff ≈ 8200 K (Simon et al 2002).…”

A test of the neutron star hypothesis for Fomalhaut b

“…non-chemically-peculiar A-type stars, only a handful have been detected in X-rays. Those rare examples display weak and very soft X-ray emission that can be described by a coronal plasma with dominant temperature components around 1 MK in the case of the A5V star beta Pictoris (Günther et al 2012) and temperature components in the range from 1-4 MK in the case of the A7V star Altair (Robrade & Schmitt 2009). Other X-ray observations of A-type stars have not led to detections (Pease et al 2006;Ayres 2008;Drake et al 2014), with the exception of HR 8799, which is listed by SIMBAD as an A6 star, but was shown by to be a peculiar type of early-F star with coronal emission, therefore not being representative for the intermediate-mass stellar regime.…”