Altair - the hottest ’cool’ star in X-rays
J. Robrade and J.H.M.M. Schmitt
Hamburger Sternwarte, Gojensbergweg 112, D-21029 Hamburg, Germany
Abstract. We present first results from a deep (130 ks) XMM-Newton observation of Altair and
study the coronal X-ray properties of a late A-type star for the first time in detail. We find that
Altair’s thin outer convective layer and its fast rotation generate a corona that resembles those of
low activity stars of later spectral type. Cool plasma at low density produces weak X-ray emission
that shows moderate variability on timescales of hours to days. We find a neon to oxygen abundance
ratio of Ne/O ∼ 0.2, similar to other inactive stars and the Sun.
Keywords: Stars: individual: Altair, Stars: activity, Stars: coronae, X-rays: stars
PACS: 95.85.Nv, 97.10.Ex, 97.10.Jb, 97.20.Ge
This conference proceeding was adapted from a poster presentation at the ’Cool
Stars 15’ conference, St. Andrews, July 2008. A full paper is in preparation.
BASIC PROPERTIES OF ALTAIR
Basic data of Altair (α Aql, HD 187642): see e.g. [1, 2, 3, 4]
• spectral type A7 IV-V (d= 5.1 pc, M= 1.8 MSun , Te f f = 7600 K)
• X-ray detected by Einstein & ROSAT (at similar low LX )
• chromospheric Lyα emission detected by IUE
• fast rotation, Vsini ≈ 220 km/s (i ≈ 60 deg, P ≈ 9 h)
• rotationally deformed (oblate, a/b ≈ 1.1)
RESULTS
Derived X-ray properties of Altair:
• X-ray emission generated by magnetic activity
• moderate (≈ 30%) variability of X-ray luminosity on timescales of hours
• corona dominated by cool plasma at 1 – 5 MK, Tav = 2.4 MK
• very low activity level: log LX /Lbol = - 7.45, log LX = 27.2 erg/s
• OVII f/i = 3.3 ± 0.9 –low density plasma, moderate radiation field
• low coronal Ne/O ratio (Ne/O ≈ 0.2), high Fe abundance
• long term stability of Altair’s corona over decades
FIGURE 1. X-ray light curve of Altair in the 0.2 – 2.0 keV band during the XMM-Newton observation
(0.5 h bins, EPIC data).
FIGURE 2. Hardness ratio (PN: 0.6-2.0 / 0.2-0.6 keV) vs. count rate. Spectral hardening during X-ray
brighter phases points to X-ray emission generated by magnetic activity.
X-ray light curves
Variability of Altair’s X-ray brightness is seen (Figure 1) at a level of roughly 30 %
on timescales of a few hours and over the total observation time. This variability could
in principle be due to rotational modulation or intrinsic variability of the X-ray emitting
features, e.g. microflaring or changing active regions. Moderate spectral hardening in
correlation with increasing count rate is present (Figure 2) and points to X-rays generated by magnetic activity. Additional variability that is most probably due to rotational
modulation appears to be present.
X-ray spectra
An example of an EPIC spectrum of Altair is shown in Figure 3, which is used to
derive its global coronal properties. Altair’s corona has an average temperature of about
FIGURE 3. EPIC PN (CCD) spectra of the two observation segments separated by roughly two weeks.
The spectra are similar and are dominated by rather cool X-ray emitting plasma.
FIGURE 4. RGS (Reflection Grating Spectrometer) spectrum of Altair with prominent X-ray lines
labelled. On the right a zoom in of the He-like triplet from OVII.
2.4 MK and its emission measure distribution (EMD) is dominated by rather cool plasma
at temperatures in the range of 1 – 4 MK, additionally a weak hotter component seems
to contribute at a few percent level. These properties are quite typical for weakly active
stars and similar to those of the quiescent Sun. This suggests that large active regions or
significant flaring components are virtually absent on the surface of Altair. Instead, the
corona appears to be dominated by quiescent regions or open magnetic field structures.
The high resolution X-ray spectrum of Altair as obtained by the RGS is shown in
Figure. 4. It exhibits several strong emission lines, which are used as diagnostics for the
X-ray emitting plasma, e.g. the
• OVII f/i ratio is sensitive to plasma density/UV-radiation
• OVIII/OVII ratio is temperature sensitive
• various neon and oxygen lines allow to derive abundance ratios
FIGURE 5. Ne/O abundance ratio of Altair (purple) and a sample of low to moderately active stars with
spectral types mid-F to mid-K (black, [5]) as well as the ’classical’ Sun (red box). Data points/regression
curves were derived from high-resolution X-ray spectra obtained by XMM-Newton and Chandra via
global fitting (diamonds/solid line) and two emission line ratios (Drake&Testa: asterisks/dotted line,
Liefke&Schmitt: squares/dashed line).
In Figure 5 we show the coronal Ne/O ratio of Altair as derived from various methods
in comparison to other low and moderately active stars of later spectral type and the Sun.
A correlation of the Ne/O ratio with coronal activity as expressed by LX /Lbol is clearly
visible and seems to be quite universal, i.e. independent of spectral type, at least for late
A-type to late K-type stars. A detailed discussion of the applied methods, results for the
other analysed stars and further references are given in [5].
ACKNOWLEDGMENTS
This work is based on observations obtained with XMM-Newton, an ESA science
mission with instruments and contributions directly funded by ESA Member States and
the USA (NASA). J.R. acknowledges support from DLR under 50QR0803.
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