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]
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
an underlying photosphere,
a linear temperature rise vs. log column mass in the chromosphere, and
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]
Theory of coronal heating
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 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.[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 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.[13]
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.[13]
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.[13] Between the generation of a magnetic field and the emission of X-rays lies the coronal heating mechanism.[13]
^ abcdSebastian, D.; Gillon, M.; Ducrot, E.; Pozuelos, F. J.; Garcia, L. J.; Günther, M. N.; Delrez, L.; Queloz, D.; Demory, B. O.; Triaud, A. H. M. J.; Burgasser, A.; De Wit, J.; Burdanov, A.; Dransfield, G.; Jehin, E.; McCormac, J.; Murray, C. A.; Niraula, P.; Pedersen, P. P.; Rackham, B. V.; Sohy, S.; Thompson, S.; Van Grootel, V. (2021). "SPECULOOS: Ultracool dwarf transit survey. Target list and strategy". Astronomy and Astrophysics. 645: 645. arXiv:2011.02069. Bibcode:2021A&A...645A.100S. doi:10.1051/0004-6361/202038827. S2CID226245978.
^Dahn, C.; Green, R.; Keel, W.; Hamilton, D.; Kallarakal, V.; Liebert, James (Sep 1985). "The Absolute Magnitude of the Flare Star AZ Cancri (LHS 2034)". Information Bulletin on Variable Stars. 2796 (1): 1–2. Bibcode:1985IBVS.2796....1D.
^Bidelman, W. P.; D. Hoffleit (1983). "The Absolute Magnitude of AZ Cancri". Information Bulletin on Variable Stars. 2414 (1): 1. Bibcode:1983IBVS.2414....1B.
^Haro G, Chavira E, Gonzalez G (Dec 1976). "Flare stars in the Praesepe field". Bol Inst Tonantzintla. 2 (12): 95–100. Bibcode:1976BITon...2...95H.
^Mullan DJ (1984). "On the possibility of resonant electrodynamic coupling in the coronae of red dwarfs". Astrophys. J. 282: 603. Bibcode:1984ApJ...282..603M. doi:10.1086/162239.
