AZ Cancri

AZ Cancri is a member of the Beehive Cluster, also known as Praesepe or NGC 2632. The spectral type of AZ Cnc is M6e,^[9] specifically M6.5Ve,^[10] and was catalogued as a flare star by Haro and Chavira in 1964 (called by them T4).^[11][12] 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^[13]

The absolute magnitude of the star has been found to be 16.9, and thus its luminosity is approximately 3.020 × 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^[14] with a radial velocity of 64.2±0.6 km/s.^[15] AZ Cancri belongs kinematically to the old disk.^[15] It is rotating at approximately 7.9±2.8 km/s.^[15]

The X-ray luminosity of AZ Cnc increased by at least two orders of magnitude during a flare that lasted more than 3 hours and reached a peak emission level of more than 10²⁹ ergs/s.^[13] 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.^[15]

The flaring atmosphere of AZ Cancri has been analysed with a stellar atmosphere model,^[16][15] and was found to consist of

1. an underlying photosphere,
2. a linear temperature rise vs. log column mass in the chromosphere, and
3. transition region (TR) with different gradients.^[15]

For the underlying photosphere, the effective temperature was found to be 2800 K, and a solar chemical composition was used.^[15] The last spectrum taken in the series after the flare was used for the quiescent chromosphere.^[15]

The line asymmetries have been attributed to downward moving material,^[15] specifically a series of flare-triggered downward moving chromospheric condensations, or chromospheric downward condensations (CDC)s as on the Sun.^[17]

The electrodynamic coupling theory of coronal heating developed in a solar context,^[18] has been applied to stellar corona.^[19] 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 L_x/L_bol, 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.^[20] Coronal heating efficiencies do not decrease toward the presumably totally convective stars near the end of the main sequence.^[13] 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%.^[13] So far there is no evidence to suggest that AZ Cnc is less efficient than more massive dwarfs in creating a corona.^[13] The saturation boundary in X-ray luminosity extends to late M dwarfs, with L_x/L_bol ~ 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.^[13]

AZ Cnc casts doubt on the applicability of electrodynamic coupling as there is no evidence for a sharp drop in L_x/L_bol when compared with other late M stars at least until subtype M8.^[13]

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.^[13] Between the generation of a magnetic field and the emission of X-rays lies the coronal heating mechanism.^[13]