AZ Cancri
Observation data Epoch J2000.0 Equinox J2000.0 (ICRS) |
|
---|---|
Constellation | Cancer |
Right ascension | 08h 40m 29.751s[1] |
Declination | +18° 24′ 09.18″[1] |
Apparent magnitude (V) | 17.59[1] |
Characteristics | |
Spectral type | M6.5eV[1] |
U−B color index | −5.1[1] |
B−V color index | 1.6[1] |
V−R color index | 1.0[1] |
R−I color index | 3.2[1] |
Variable type | UV[2] |
Astrometry | |
Proper motion (μ) | RA: −799[1] mas/yr Dec.: −463[1] mas/yr |
Parallax (π) | 71.1 ± 1[1] mas |
Distance | 45.9 ± 0.6 ly (14.1 ± 0.2 pc) |
Absolute magnitude (MV) | 16.85[3] |
Other designations | |
Database references | |
SIMBAD | data |
AZ Cancri (AZ Cnc) is a M-type flare star in the constellation Cancer.[1] It has an apparent visual magnitude of approximately 17.59.[1]
Contents
Observations
AZ Cancri is a member of the Beehive Cluster, also known as Praesepe or NGC 2632. The spectral type of AZ Cnc is M6e,[4] specifically M6.5Ve,[5] and was catalogued as a flare star by Haro and Chavira in 1964 (called by them T4).[6][7] AZ Cnc has also been found to be an x-ray source, with the ROSAT designations of RX J0840.4+1824 and 1RXS J084029.9+182417. The X-ray luminosity has been found to be 27.40 ergs/s[8]
Physical characteristics
The absolute magnitude of the star has been found to be 16.9, and thus its luminosity is approximately 3.020 x 1030 ergs/s.[citation needed]
AZ Cancri is located approximately 14.0 parsecs (46 ly) from the Sun, and is considered a very low mass star [9] with a radial velocity of 64.2±0.6 km/s.[10] AZ Cancri belongs kinematically to the old disk.[10] It is rotating at approximately 7.9±2.8 km/s.[10]
Flaring
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The X-ray luminosity of AZ Cnc increased by at least two orders of magnitude during a flare that lasted more than 3 h and reached a peak emission level of more than 1029 ergs/s.[8] During another long duration flare (March 14, 2002) on AZ Cnc, very strong wing asymmetries occurred in all lines of the Balmer series and all strong He I lines, but not in the metal lines.[10]
The flaring atmosphere of AZ Cancri has been analysed a with stellar atmosphere model,[11][10] and was found to consist of
- an underlying photosphere,
- a linear temperature rise vs. log column mass in the chromosphere, and
- transition region (TR) with different gradients.[10]
For the underlying photosphere, the effective temperature was found to be 2800 K, and a solar chemical composition was used.[10] The last spectrum taken in the series after the flare was used for the quiescent chromosphere.[10]
The line asymmetries have been attributed to downward moving material,[10] specifically a series of flare-triggered downward moving chromospheric condensations, or chromospheric downward condensations (CDC)s as on the Sun.[12]
Theory of coronal heating
The electrodynamic coupling theory of coronal heating developed in a solar context,[13] has been applied to stellar coronae.[14] A distinctive feature is the occurrence of a resonance between the convective turnover time and the crossing time for Alfvén waves in a coronal loop. The resonance attains a maximum among the early M dwarf spectral types and declines thereafter. A turnover in coronal heating efficiency, presumably manifested by a decrease in Lx/Lbol, becomes evident toward the late M spectral types when the theory is applicable. This is consistent with an apparent lack of X-ray emission among the late M dwarfs.[15] Coronal heating efficiencies do not decrease toward the presumably totally convective stars near the end of the main sequence.[8] For "saturated" M dwarfs, 0.1% of all energy is typically radiated in X-rays, while for AZ Cnc this number increases during flaring to 7%.[8] So far there is no evidence to suggest that AZ Cnc is less efficient than more massive dwarfs in creating a corona.[8] The saturation boundary in X-ray luminosity extends to late M dwarfs, with Lx/Lbol ~ 10−3 for saturated dwarfs outside flaring. No coronal dividing line exists in the Hertzsprung–Russell diagram at the low-mass end of the main sequence.[8]
AZ Cnc casts doubt on the applicability of electrodynamic coupling as there is no evidence for a sharp drop in Lx/Lbol when compared with other late M stars at least until subtype M8.[8]
Dynamo
AZ Cnc has a corona and this may indicate that a distributive dynamo is just as efficient in producing magnetic flux as a shell dynamo.[8] Between the generation of a magnetic field and the emission of X-rays lies the coronal heating mechanism.[8]
References
- ↑ 1.00 1.01 1.02 1.03 1.04 1.05 1.06 1.07 1.08 1.09 1.10 1.11 1.12 1.13 V* AZ Cnc -- Flare Star, database entry, SIMBAD. Accessed on line October 13, 2010.
- ↑ AZ Cnc, database entry, The combined table of GCVS Vols I-III and NL 67-78 with improved coordinates, General Catalogue of Variable Stars, Sternberg Astronomical Institute, Moscow, Russia. Accessed on line October 13, 2010.
- ↑ From apparent magnitude and parallax.
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