AZ Cancri

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]

AZ Cancri

Close-up of the SDSS DR6 image with AZ Cnc centered.
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
Distance45.9 ± 0.6 ly
(14.1 ± 0.2 pc)
Absolute magnitude (MV)16.85[3]
Other designations
AZ Cnc, GJ 316.1, LHS 2034, NLTT 20016.[1]
Database references
SIMBADdata

Observations
Image of AZ Cancri from the Sloan Digital Sky Survey; it is the red star close to the centre.

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.

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
See also: flare star

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 with a stellar atmosphere model,[11][10] 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.[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 corona.[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]

gollark: No it doesn't.
gollark: It hides dark secrets.
gollark: https://elm-lang.org
gollark: Well, it's nice and functional, and the core library is nice.
gollark: Basically.

References
  1. "V* AZ Cnc". SIMBAD. Centre de données astronomiques de Strasbourg. Retrieved October 13, 2010.
  2. 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.
  3. From apparent magnitude and parallax.
  4. Kirkpatrick JD, Henry TJ, McCarthy D (1991). "A standard stellar spectral sequence in the red/near-infrared - Classes K5 to M9". Astrophys. J. Suppl. Ser. 77: 417. Bibcode:1991ApJS...77..417K. doi:10.1086/191611.
  5. Dahn C, Green R, Keel W, Hamilton D, Kallarakal V, Liebert J (Sep 1985). "The Absolute Magnitude of the Flare Star AZ Cancri (LHS 2034)". Information Bull Var Stars. 2796 (9): 1–2. Bibcode:1985IBVS.2796....1D.
  6. Bidelman, W. P.; D. Hoffleit. "The Absolute Magnitude of AZ Cancri". Information Bulletin on Variable Stars. 2414 (1). Bibcode:1983IBVS.2414....1B.
  7. 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.
  8. Fleming TA; Giampapa MS; Schmitt JHMM; Bookbinder JA (Jun 1993). "Stellar coronae at the end of the main sequence - A ROSAT survey of the late M dwarfs". Astrophys. J. 410 (1): 387–92. Bibcode:1993ApJ...410..387F. doi:10.1086/172755.
  9. Monet DG, Dahn CC, Vrba FJ, Harris HC, Pier JR, Luginbuhl CB, Ables HD (1992). "U.S. Naval Observatory CCD parallaxes of faint stars. I - Program description and first results". Astron. J. 103: 638. Bibcode:1992AJ....103..638M. doi:10.1086/116091.
  10. Fuhrmeister B; Schmitt JHMM; Hauschildt PH (Jun 2005). "Detection of red line asymmetries in LHS 2034". Astron. Astrophys. 436 (2): 677–86. Bibcode:2005A&A...436..677F. doi:10.1051/0004-6361:20042518.
  11. Hauschildt PH, Allard F, Baron E (Feb 1999). "The NextGen Model Atmosphere Grid for 3000<=T_eff<=10,000 K". Astrophys. J. 512 (1): 377–85. arXiv:astro-ph/9807286. Bibcode:1999ApJ...512..377H. doi:10.1086/306745.
  12. Fisher GH (Nov 1989). "Dynamics of flare-driven chromospheric condensations". Astrophys. J. 346 (11): 1019–29. Bibcode:1989ApJ...346.1019F. doi:10.1086/168084.
  13. Ionson J (1984). "A unified theory of electrodynamic coupling in coronal magnetic loops - The coronal heating problem". Astrophys. J. 276: 357. Bibcode:1984ApJ...276..357I. doi:10.1086/161620.
  14. 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.
  15. Bookbinder, J. A. (1985). Observations of non-thermal radiation from late-type stars (Thesis). Cambridge, MA: Harvard University. Bibcode:1985PhDT........13B.
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