Single X-ray star

Each single X-ray star that is close by offers a valuable opportunity. The benefit of studying single stars is that it allows measurements free of any effects of a companion or being a part of a multiple star system. Theories or models can be more readily tested. In addition to the Sun, there are many solitary, or unary stars or star systems throughout the galaxy that emit X-rays. "Stars nearest the Sun play an especially important role in understanding the physical phenomena in stellar outer atmospheres, since their proximity permits detection of even low levels of activity." bound by gravitational attraction. The term 'star system' may also refer to a system of a single star together with a planetary system of orbiting smaller bodies.
A single star is usually regarded separately or as distinct from others in a group, as an individual star rather than part of a pair, a multiple-star system or a group such as a star cluster.
Solitary star
A solitary star differs from a single star in that the former exists alone, secluded or isolated from other stars. For example, Psi Aquarii (93 Aquarii) is a solitary star. Radial velocity measurements have not yet revealed the presence of planets orbiting it.
Unary star
A unary star contrasts with a binary star, trinary (three stars), and a multiple star. It is not necessarily alone like the solitary star.
Binary analog
Stellar companions can have profound consequences for dynamical evolution. However, in looking for binary analogs of single stars there are sound reasons to retain some wide binary systems for study:
# many visual binary systems have not been confirmed as physical systems (by observations of common proper motion) and may actually be spurious optical pairs,
# the properties of a very high mass or very low mass coeval companion may constrain the age of the system,
# wide binaries (those with semimajor axes ~10 AU) are weakly bound systems where the tidal field of the Galactic disk, perturbing effects of passing stars and molecular clouds, may disrupt an appreciable fraction of these systems over the main sequence lifetime of solar-mass stars.
Binary systems with semimajor axes of < 1000 AU are very unlikely to resemble a single-star system. After detecting X-ray photons from the Sun in the course of the rocket flight, T. Burnight wrote, “The sun is assumed to be the source of this radiation although radiation of wave-length shorter than 4 angstroms would not be expected from theoretical estimates of black body radiation from the solar corona.”
ε Eridani
Epsilon Eridani is visual spectral type K2V, and it is only 10.5 lyr away. It is one of the nearest Sun-like stars and it is a single star. Ca II emission is variable in ε Eri. ε Eri is a strong X-ray star. Spectral profiles of Zeeman broadening yield a magnetic field of 2390±424 G with a filling factor of 0.49±0.17. The abundance variations between the photosphere, chromosphere and corona, can be interpreted in terms of the evolution of emerging magnetic fields rather than the topology of these magnetic fields: newly emerging features exhibit photospheric compositions (small compact structures with tightly closed field lines) that gradually evolve to become more open structures with a relative enrichment of low first ionization potential (FIP) elements, such as is found in ε Eri.
Magnetic activity of ε Eridani
Epsilon Eridani has a higher level of magnetic activity than the Sun, with a stellar wind 30 times as strong.
Chromosphere of ε Eridani
The chromosphere of Epsilon Eridani is more magnetically active than the Sun's.
Photosphere of ε Eridani
Approximately 9% of the deep photosphere is found to have a magnetic field with a strength about 0.14 teslas.
ε Eridani as a BY Draconis variable
Its rotation period is a relatively rapid 11.1 d, although this varies by latitude. The mean rotation as compared with the Sun is shorter at 11.68 d. Stars that vary in magnitude because of magnetic activity coupled with rotation are classified as BY Draconis variables. Observations have shown this star to vary as much as 0.050 in magnitude due to starspots and other short-term magnetic activity.
Corona of ε Eridani
Relative to the Sun, the outer atmosphere of Epsilon Eridani appears both larger and hotter. As a result of a systematic study of every known dwarf type K or M star within 6 pc of the Sun, all late-type K and M dwarf stars are probably surrounded by hot X-ray-emitting coronae. The existence of a stellar corona seems to be essentially independent of the detailed characteristics of the underlying star.
The coronal temperatures of ε Eri fit two-components of log T1 5.95 ± 0.4 (891,251 K) and log T2 6.95 ± 0.2 (8,912,509 K). A coronal filling factor of less than one is consistent with the additional presence of active regions. And, it is nearby at about 24.4 lyr.
Sun
The Sun is one of many unary stars throughout the galaxy that emits X-rays. The benefit of studying single stars is that it allows measurements free of any effects of a companion or being a part of a multiple star system. Theories or models can be more readily tested. Solar analog stars would have an effective surface temperature of ~5800 K. , Solar Analog, and Solar Twin stars are particularly similar to the Sun, with solar twin being more similar than solar analog, and solar-type being less similar than solar analog. Observations of these stars are important for a better understanding of the properties of our own Sun in relation to other stars and the habitability of planets. Similarity to the Sun allows for checking derived quantities — like temperature, from the color index — against the Sun, the only star whose temperature is confidently known. Solar-type stars show highly correlated behavior between their rotation rates and their chromospheric activity (e.g. Calcium H & K line emission) and coronal activity (e.g. X-ray emission). Solar analogs are photometrically similar to the Sun with the following qualities:
# temperature within 500 K solar (roughly 5200 to 6300 K),
# metallicity of 50-200% solar, i.e., the star's proplyd has similar amounts of dust, and
# no close companion (orbital period ≤ 10 d).
===Solar "binary" analogs===
Notwithstanding speculation on the existence of a "Nemesis" companion, stellar companions can have profound consequences for dynamical evolution. However, in looking for solar analogs there are sound reasons to retain some wide binary systems for study:
# many visual binary systems have not been confirmed as physical systems (by observations of common proper motion) and may actually be spurious optical pairs,
# the properties of a very high mass or very low mass coeval companion may constrain the age of the system,
# wide binaries (those with semimajor axes ~10 AU) are weakly bound systems where the tidal field of the Galactic disk, perturbing effects of passing stars and molecular clouds, may disrupt an appreciable fraction of these systems over the main sequence lifetime of solar-mass stars. Since spectroscopic measurements of metallicities are not available for most stars, the luminosity evolution of the constant-mass Sun has been empirically described using
: L/Lsolar = [1 + 0.4(1 - t/t0)] ,
where t/t0 is the ratio of the age to the present age of 4.6 Gyr. This corresponds approximately to a range of spectral types G0 through K1. sufficiently so that coronal X-ray emission is an appropriate stellar chronometer, although the age of any particular G star cannot be accurately derived from its X-ray emission.
A distance- and stellar radius-independent parameter used to describe X-ray emission is Rx:
: Rx = log10(Lx/Lbol).
For G stars in open clusters of ~50 Myr (e.g. the Pleiades) Rx is near -3, and monotonically decreases with age.
Young solar analogs
Young solar analogs can be selected on the basis of
# distances < 25 pc,
# lack of known compaions with a projected distance of 800 AU,
# bolometric luminosities in the range -0.37 < log10(L/Lsolar) < 0.06 and below the Hyades ZAMS for the star's effective temperature, and
# Rx > -4.86. To understand what's going on in Vega it is important to realize that the coronal magnetic field plays an integral part in the energy balance of coronal plasma. An extensive convection zone is not required, and any star with magnetic field strengths and geometry similar to the Sun's will possess a corona.
Betelgeuse
Betelgeuse is a semiregular variable star located approximately 640 light-years (lyr) from the Sun. Betelgeuse appears to be always X-ray dark. The X-ray flux from the entire stellar surface corresponds to a surface flux limit that ranges from 0.03 to 7 W m (30 to 7000 erg s cm ) at T 1 MK, to about 1 mW m (1 erg s cm ) at higher temperatures, five orders of magnitude below the quiet Sun X-ray surface flux.<ref namePosson/> But, due to instrumental limitations of Chandra the presence of low-level emission on the scale of coronal holes cannot be ruled out.<ref name=Posson/> Betelgeuse is a red supergiant, and one of the largest and most luminous stars known in the visible range.

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