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HD 984

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Coordinates: Sky map 00h 14m 10s, −07° 11′ 57″
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HD 984

HD 984 is the brightest star in this Sloan Digital Sky Survey image. The spiral galaxy NGC 47 can be seen at upper left.
Observation data
Epoch J2000      Equinox J2000
Constellation Cetus[note 1]
Right ascension 00h 14m 10.25s[1]
Declination −07° 11′ 56.82″[1]
Apparent magnitude (V) 7.32[2]
Characteristics
A
Evolutionary stage F-type main-sequence star[3]
Spectral type F7V[3]
B−V color index 0.522±0.01[4]
B
Evolutionary stage Brown dwarf[4][5]
Spectral type M6.5±1.5[6]
Apparent magnitude (J) 13.28±0.06[6]
Apparent magnitude (H) 12.6±0.05[6]
Astrometry
Radial velocity (Rv)1.04±0.25[1] km/s
Proper motion (μ) RA: 104.775 mas/yr[1]
Dec.: −68.016 mas/yr[1]
Parallax (π)21.877 ± 0.0249 mas[1]
Distance149.1 ± 0.2 ly
(45.71 ± 0.05 pc)[1]
Absolute magnitude (MV)3.95±0.07[4]
Orbit[5]
PrimaryA
CompanionB
Period (P)140+50
−30 yr
Semi-major axis (a)28+7
−4 AU
Eccentricity (e)0.76±0.05
Inclination (i)120.8+1.8
−1.6°
Details
A
Mass1.2±0.06[4] M☉
Radius1.247±0.053[4] R☉
Luminosity2.218[6] L☉
Surface gravity (log g)4.38±0.06[3] cgs
Temperature6326±80[3] K
Metallicity [Fe/H]−0.01±0.12[3] dex
Rotation1.39±0.05[7] d
Rotational velocity (v sin i)42.13±1.65[4] km/s
Age30–200[4] Myr
B
Mass61±4[5] MJup
Radius0.162[6][a] R☉
Luminosity1.318×10−3[6] L☉
Temperature2730+120
−180[6] K
Metallicity [Fe/H]−0.62±0.02[3][note 2] dex
Rotational velocity (v sin i)12.72+0.03
−0.02[3] km/s
Position (relative to A)[4]
ComponentB
Epoch of observation2015
Angular distance201.6±0.4 mas
Position angle92.2±0.5°
Other designations
BD−08 24, Gaia DR2 2431157720981843200, Gaia DR3 2431157720981843200, HIP 1134, SAO 128650, PPM 182008, TIC 408012676, TYC 4670-773-1, GSC 04670-00773, 2MASS J00141025-0711569
Database references
SIMBADdata

HD 984 is a F-type main-sequence star located in the equatorial constellation Cetus. It is a young star, estimated 30 to 200 million years old, and is orbited by a brown dwarf companion. Parallax measurements by the Gaia spacecraft imply a distance of 150 light-years (46 pc) to HD 984. At an apparent magnitude of 7.32, the star is too dim to be visible to the naked eye.

Characteristics[edit]

The apparent magnitude of HD 984, i.e. its brightness relative to Earth, is 7.32.[2] Such brightness is lower than the limit for naked-eye visibility, generally defined as 6.5m, meaning that HD 984 can't be seen with the naked eye. It may be visible by a small telescope[8] or binoculars instead.[9] The absolute magnitude, i.e. the magnitude of HD 984 if it was seen at 10 parsecs (32.6 ly), is 3.95.[4] A parallax measurement by the Gaia spacecraft (21.9 mas) imply a distance of 149 light-years.[1] Kinematically, the system it is part of the Columba group.[4][10] It has a relatively high proper motion.[4]

The star is visually close (57") to BD-08 25,[11] which is a star of apparent magnitude 9.14 unrelated to the system, being 1,800 light-years (550 pc) away from Earth based on its parallax.[12] The galaxy NGC 47 lies just 320" from HD 984.[11]

Age estimates[edit]

Based on the assumption that HD 984 is part of the Columba group, the stellar age would be of 30 Myr. However, there is still a possibility that HD 984 is a kinematic interloper or the Columba group is not sufficiently characterized to reliably assign an age. Previous isochronal ages of <0.48, 1.2+0.7
−0.9 and 3.1+1
−1.6 Gyr have been given by earlier studies. Since HD 984 A is a main sequence star, all isochronal ages will have high uncertainty.[4]

An analysis by HD 984 B's discovery team say that the system is likely to be less than 200 million years, based on HD 984 A's rotation and stellar activity. The same research also says that ages less than 30 million years can be ruled out, based on isochronal age constraints for HD 984 B. Therefore, the age is very likely between 30 and 200 million years.[4]

HD 984 A[edit]

HD 984 A is has a spectral classification F7V, matching a F-type star in the main sequence.[2][4] It has 1.2 times the mass and 1.25 times the radius of the Sun.[4] Its surface has an effective temperature of 6,326 K (6,053 °C),[3] 464 degrees hotter than the Sun, and its luminosity is equivalent to 2.21 times the solar luminosity.[6] HD 984 A is one of the fastest-rotating stars, with a rotation period estimated to be less than 1.6 days (38 hours), faster than 99% of all stars.[3] The Sun's rotational period is 27 days for comparison.

Its rotation, coronal activity and cromospheric activity indicates that it is a young and active main-sequence star. Its age is very likely to less than 200 millon years, based on its rotation and activity indicators. The star is expected to live 5 billion years on the main sequence.[4]

HD 984 B[edit]

HD 984 B is a brown dwarf, a class of astronomical objects that are intermediate between planets and stars, having masses between 13 and 75 MJ, and, unlike stars, can't produce thermonuclear fusion of hydrogen.[13] It was discovered in 2015 by Meshkat et al., using direct imaging from the Apodizing Phase Plate coronagraph at the Very Large Telescope with NaCo.[4]

Orbit[edit]

HD 984 B is orbiting HD 984 A at a semi-major axis of 28 AU (4.2×109 km), which is about the same distance of Neptune to the Sun. It has an orbital period of 140 years and an eccentric orbit (e = 0.76±0.05). This eccentricity causes its distance to HD 984 to vary from 6.72 to 49.3 AU.[b] HD 984 B was initially thought to have a low eccentricity in orbit (e = 0.23+0.11
−0.23), although the data was uncertain. This discrepancy is caused by increased orbit coverage and astrometric acceleration used in the novel value. The orbital inclination is of 121 degrees.[5]

HD 984 B is expected to be in periastron in 2028.[5]

Physical parameters[edit]

Its mass was firstly estimated using the age of the system by Meshkat et al. (2015) and Jonson-Groh et al. (2017).[6][4] Using this method, masses ranging from 34 MJ to 94 MJ are obtained, assuming ages of 30 and 200 million years respectively.[6] Assuming the largest mass and age, HD 984 B would be in the stellar mass regime. A 2022 study found a dynamical mass of 61±4 MJ, which places HD 984 B in the brown dwarf regime at 99.7% confidence. This mass was obtained using astrometric acceleration, new direct imaging of the object and radial velocity measurements, and is consistent with evolutionary models.[5]

HD 984 B is a hot brown dwarf[3] and has a spectral class of M6.5, with an uncertainty of 1.5. Having this spectral type, HD 984 B has an effective temperature of 2730 K.[6] At a luminosity of about 1.318×10−3 L☉, this corresponds to a radius of 0.162 R☉ (1.576 RJ) based on the Stefan–Boltzmann law.[a] Its metallicity, i.e. its abundance of elements other than hydrogen and helium, is −0.62±0.02 dex, just one-fourth of the solar metalicity (0 dex), altough this value might be biased. This brown dwarf more likely formed via gravitational collapse or disk instability rather than core accretion.[3]

See also[edit]

Notes[edit]

  1. ^ a b Applying the Stefan–Boltzmann law with a nominal solar effective temperature of 5,772 K:
    .
  2. ^ Calculated using the semi-major axis and the orbital eccentricity.
    28×(1-0.76) = 6.72 AU.
    28×(1+0.76) = 49.28 AU.
  1. ^ Obtained with a right ascension of 00h 14m 10.25s and a declination of −07° 11′ 56.82″[1] on this website.
  2. ^ This value might be biased since it has been derived only using observations in the K band, which isn't is the best band to derive a metallicity.

References[edit]

  1. ^ a b c d e f g h Vallenari, A.; et al. (Gaia collaboration) (2023). "Gaia Data Release 3. Summary of the content and survey properties". Astronomy and Astrophysics. 674: A1. arXiv:2208.00211. Bibcode:2023A&A...674A...1G. doi:10.1051/0004-6361/202243940. S2CID 244398875. Gaia DR3 record for this source at VizieR.
  2. ^ a b c "HD 984". SIMBAD. Centre de données astronomiques de Strasbourg.
  3. ^ a b c d e f g h i j k Costes, J. C.; Xuan, J. W.; Vigan, A.; Wang, J.; D'Orazi, V.; Mollière, P.; Baker, A.; Bartos, R.; Blake, G. A. (2024-04-17), Fresh view of the hot brown dwarf HD 984 B through high-resolution spectroscopy, arXiv:2404.11523
  4. ^ a b c d e f g h i j k l m n o p q r Meshkat, T.; Bonnefoy, M.; Mamajek, E. E.; Quanz, S. P.; Chauvin, G.; Kenworthy, M. A.; Rameau, J.; Meyer, M. R.; Lagrange, A.-M.; Lannier, J.; Delorme, P. (2015-11-01). "Discovery of a Low-Mass Companion to the F7V star HD 984". Monthly Notices of the Royal Astronomical Society. 453 (3): 2379–2387. arXiv:1507.08291. doi:10.1093/mnras/stv1732. ISSN 0035-8711.
  5. ^ a b c d e f Franson, Kyle; Bowler, Brendan P.; Brandt, Timothy D.; Dupuy, Trent J.; Tran, Quang H.; Brandt, G. Mirek; Li, Yiting; Kraus, Adam L. (2022-02-01). "Dynamical Mass of the Young Substellar Companion HD 984 B". The Astronomical Journal. 163 (2): 50. arXiv:2111.01803. Bibcode:2022AJ....163...50F. doi:10.3847/1538-3881/ac35e8. ISSN 0004-6256.
  6. ^ a b c d e f g h i j k Johnson-Groh, Mara; Marois, Christian; De Rosa, Robert J.; Nielsen, Eric L.; Rameau, Julien; Blunt, Sarah; Vargas, Jeffrey; Ammons, S. Mark; Bailey, Vanessa P.; Barman, Travis S.; Bulger, Joanna; Chilcote, Jeffrey K.; Cotten, Tara; Doyon, René; Duchêne, Gaspard (2017-04-01). "Integral Field Spectroscopy of the Low-mass Companion HD 984 B with the Gemini Planet Imager". The Astronomical Journal. 153 (4): 190. arXiv:1703.02607. Bibcode:2017AJ....153..190J. doi:10.3847/1538-3881/aa6480. ISSN 0004-6256.
  7. ^ Bowler, Brendan P.; Tran, Quang H.; Zhang, Zhoujian; Morgan, Marvin; Ashok, Katelyn B.; Blunt, Sarah; Bryan, Marta L.; Evans, Analis E.; Franson, Kyle; Huber, Daniel; Nagpal, Vighnesh; Wu, Ya-Lin; Zhou, Yifan (2023-04-01). "Rotation Periods, Inclinations, and Obliquities of Cool Stars Hosting Directly Imaged Substellar Companions: Spin–Orbit Misalignments Are Common". The Astronomical Journal. 165 (4): 164. arXiv:2301.04692. Bibcode:2023AJ....165..164B. doi:10.3847/1538-3881/acbd34. ISSN 0004-6256.
  8. ^ "★ HD 984". Stellar Catalog. Retrieved 2024-06-16.
  9. ^ Macrobert, Alan. "Binoculars: Halfway to a Telescope". Sky & Telescope. American Astronomical Society. Retrieved June 16, 2024.
  10. ^ Nielsen, E. L.; De Rosa, R.; Macintosh, B.; Wang, J.; Ruffio, J.; Chiang, E.; Marley, M.; Saumon, D.; Savransky, D.; Gemini Planet Imager Exoplanet Survey Team (2020-01-01). "The Gemini Planet Imager Exoplanet Survey: Giant Planet and Brown Dwarf Demographics from 10-100 AU". American Astronomical Society Meeting Abstracts #235. 235: 280.02. Bibcode:2020AAS...23528002N.
  11. ^ a b "coord 00 14 10.2528328224-07 11 56.812684848 (ICRS, J2000, 2000.0), radius: 7 arcmin". simbad.cds.unistra.fr. Retrieved 2024-06-15.
  12. ^ "BD-08 25". SIMBAD. Centre de données astronomiques de Strasbourg. Retrieved June 15, 2024.
  13. ^ "Brown dwarf | Astronomy, Formation & Characteristics | Britannica". www.britannica.com. 2024-04-22. Retrieved 2024-04-24.


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