AZ Cancri

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

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]

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

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

  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 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. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 V* AZ Cnc -- Flare Star, database entry, SIMBAD. Accessed on line 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. 1 2 3 4 5 6 7 8 9 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". Ap 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. 1 2 3 4 5 6 7 8 9 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". Ap J. 512 (1): 377–85. Bibcode:1999ApJ...512..377H. arXiv:astro-ph/9807286 Freely accessible. doi:10.1086/306745.
  12. ↑ Fisher GH (Nov 1989). "Dynamics of flare-driven chromospheric condensations". Ap 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". Ap 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". Ap 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. Retrieved 8 December 2014.
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