View source for HD 984

{{Short description|F-type star in the constellation Cetus}}
{{Starbox begin}}
{{Starbox image
| image         =[[File:HD 984 in SDSS.png|240px]]
| caption       = HD 984 is the brightest star in this [[Sloan Digital Sky Survey]] image. The spiral galaxy [[NGC 47]] can be seen at upper left.
}}
{{Starbox observe
| epoch         = J2000
| equinox       = J2000
| constell      = [[Cetus]]{{Efn|Obtained with a right ascension of {{RA|00|14|10.25}} and a declination of {{DEC|-07|11|56.81}}<ref name="GaiaDR3"/> on this [https://djm.cc/constellation.html website].|name=Constellation|group=note}}
| ra            = {{RA|00|14|10.25283}}<ref name="GaiaDR3"/>
| dec           = {{DEC|-07|11|56.8127}}<ref name="GaiaDR3"/>
| appmag_v      = 7.32<ref name="SIMBAD"/>
}}
{{Starbox character
| component1    = A
| type          = [[Main sequence]]<ref name=Costes2024/>
| class         = F7V<ref name=Costes2024/>
| b-v           = {{Val|0.522|0.01}}<ref name=Meshkat2015/>
| component2    = B
| type2         = [[Brown dwarf]]<ref name=Meshkat2015/><ref name=Franson2022/>
| class2        = M6.5{{±|1.5}}<ref name=Johnson-Groh2017/>
| appmag_1_passband2 = J
| appmag_1_2    = {{Val|13.28|0.06}}<ref name=Johnson-Groh2017/>
| appmag_2_passband2 = H
| appmag_2_2    = {{Val|12.6|0.05}}<ref name=Johnson-Groh2017/>
| appmag_3_passband3 = K
| appmag_3_2    = {{val|12.2|0.04}}<ref name=Franson2022/>
}}
{{Starbox astrometry
| radial_v       = {{Val|1.04|0.25}}<ref name="GaiaDR3"/>
| prop_mo_ra    = 104.775
| prop_mo_dec   = {{val|-68.016}}
| pm_footnote   = <ref name="GaiaDR3"/>
| parallax      = 21.877
| p_error       = 0.0249
| parallax_footnote = <ref name="GaiaDR3"/>
| dist_footnote = <ref name="GaiaDR3"/>
| absmag_v      = {{Val|3.95|0.07}}<ref name=Meshkat2015/>
}}
{{Starbox orbit
| reference     = <ref name=Franson2022/>
| primary       = A
| name          = B
|period         = {{Val|140|50|30}}
| inclination   ={{Val|120.8|1.8|1.6}}
| eccentricity  = {{Val|0.76|0.05}}
| axis_unitless ={{Val|28|7|4|ul=AU}}
}}
{{Starbox detail
| component1    = A
| mass          = {{Val|1.2|0.06}}<ref name=Meshkat2015/> 
| radius        = {{Val|1.247|0.053}}<ref name=Meshkat2015/> 
| luminosity    = {{val|2.218|0.142|0.134}}<ref name=Johnson-Groh2017/>
| temperature   = {{Val|6326|80}}<ref name=Costes2024/> 
| gravity       = {{val|4.38|0.06}}<ref name=Costes2024/> 
| metal_fe      = {{Val|-0.01|0.12}}<ref name=Costes2024/> 
| rotational_velocity = {{Val|42.13|1.65}}<ref name=Meshkat2015/> 
| rotation      = {{Val|1.39|0.05}}<ref name=Bowler2023/> days
| age_myr       = 30–200<ref name=Meshkat2015/> 
<!-- HD 984 B -->
| component2   = B
| mass_mj2     = {{Val|61|4}}<ref name=Franson2022/>
| radius2      = {{Solar radius calculator|type=logLT|-2.88|2730|decimals=3}}<ref name=Franson2022/>{{efn | name=radius | Applying the [[Stefan–Boltzmann law]] with a nominal [[sun|solar]] [[effective temperature]] of 5,772&nbsp;[[Kelvin|K]]: 
:<math>\sqrt{\biggl(\frac{5,772}{2,730}\biggr)^4 \cdot 10^{-2.88}} = 0.162\ R_\odot</math>.}}
| luminosity2  = {{Val|1.318|0.094|0.088|e=-3}}<ref name=Franson2022/> 
| temperature2 = {{Val|2730|120|180}}<ref name=Johnson-Groh2017/> 
| rotational_velocity2 = {{val|12.72|0.03|0.02}}<ref name=Costes2024/> 
| metal_fe2    = {{Val|-0.62|0.02}}<ref name=Costes2024/>{{efn|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.|group=note}}
}}
{{Starbox relpos
| reference    = <ref name=Meshkat2015/>
| component    = B
| primary      = A
| epoch        = 2015
| angdistmas   = {{Val|201.6|0.4}}
| posang       = {{val|92.2|0.5}}
}}
{{Starbox catalog
| names       = {{odlist|BD=-08 24|GSC=04670-00773|HIP=1134|2MASS=J00141025-0711569|Gaia DR3=2431157720981843200|Gaia DR2=2431157720981843200|TIC=408012676|TYC=4670-773-1|SAO=128650|PPM=182008}}
}}
{{Starbox reference
|Simbad=HD+984
}}
{{Starbox end}}
'''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)|Gaia]] spacecraft imply a distance of {{convert|150|ly|pc}} to HD 984. At an [[apparent magnitude]] of 7.32, the star is too dim to be visible to the [[naked eye]].

== Characteristics ==
The [[apparent magnitude]] of HD 984, i.e. its brightness relative to Earth, is 7.32.<ref name=SIMBAD/> Such brightness is lower than the limit for naked-eye visibility, generally defined as 6.5<sup>m</sup>, meaning that HD 984 can't be seen with the [[naked eye]]. It may be visible by a [[small telescope]]<ref name="Stellar Catalog"/> or [[binoculars]] instead.<ref name=binoculars/> The absolute magnitude, i.e. the magnitude of HD 984 if it was seen at {{Convert|10|pc|ly|sigfig=3}}, is 3.95.<ref name=Meshkat2015/> [[Stellar kinematics|Kinematically]], the system it is part of the [[Columba group]].<ref name=Meshkat2015/><ref name=Nielsen2020/> It has a relatively high [[proper motion]].<ref name=Meshkat2015/>

The star is visually close (57") to BD-08 25,<ref name=Coordinates/> which is a star of apparent magnitude 9.14 unrelated to the system, being {{Convert|1800|ly|pc}} away from Earth based on its parallax.<ref>{{Cite simbad|title=BD-08 25|access-date=June 15, 2024}}</ref> The galaxy [[NGC 47]] lies just 320" from HD 984.<ref name=Coordinates/>

=== Age estimates ===
Based on the assumption that HD 984 is part of the Columba group, the stellar age would be of 30{{nbsp}}[[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 [[Stellar isochrone|isochronal]] ages of {{Val|0.48|p=<}}, {{Val|1.2|0.7|0.9}} and {{Val|3.1|1|1.6|ul=Gyr}} have been given by earlier studies. Since HD 984 A is a [[main sequence star]], all isochronal ages will have high uncertainty.<ref name=Meshkat2015/> 

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.<ref name=Meshkat2015/>

== HD 984 A ==
HD 984 A is a [[F-type main-sequence star]], [[Astronomical spectroscopy|spectroscopically]] matching a class F7V.<ref name=Costes2024/> Those stars are typically larger, hotter and brighter than the Sun, and fuse [[hydrogen]] into [[helium]] at their [[Stellar core|core]]. HD 984 A is no expection, being 20% more massive than the Sun, 25% larger and over two times brighter.<ref name=Meshkat2015/><ref name=Johnson-Groh2017/> Its surface has an [[effective temperature]] of {{Convert|6326|K|C|lk=in}},<ref name=Costes2024/> giving it a yello-white hue typical of [[F-type star]]s.<ref name=csiro/> HD 984 A rotates faster than 99% of all stars, with a [[Rotation period (astronomy)|rotation period]] estimated to be less than {{Convert|1.6|day|hour|abbr=off}}.<ref name=Costes2024/> The Sun's rotational period is 27 days for comparison. 

Its rotation, [[Stellar corona|coronal]] activity and [[Chromosphere|cromospheric]] activity indicates that it is a young and active main-sequence star, likely less than 200 million years old. It is expected to live 5 billion years on the [[main sequence]].<ref name=Meshkat2015/> After that, it will cease its [[hydrogen]] at its [[stellar core|core]] and evolve into a [[red giant]] star, increasing in size and luminosity, while decreasing in temperature.

== HD 984 B ==
HD 984 B is a [[brown dwarf]], a class of astronomical objects that are intermediate between [[Planet|planets]] and [[Star|stars]], having masses between {{Jupiter mass|13 and 75|link=y}}, and, unlike stars, can't produce [[thermonuclear fusion]] of hydrogen.<ref name=Britannica/> 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.<ref name=Meshkat2015/>

=== Orbit ===
HD 984 B takes 140 years to complete an orbit around HD 984 A, about the same as [[Neptune]]. The orbit's [[semi-major axis]] is of {{Convert|28|AU|km|abbr=out}}, also similar to Neptune. Its orbit is highly-[[orbital eccentricity|eccentric]] {{Nowrap|1=(e = {{val|0.76|0.05}})}}, meaning that its orbital distance vary from 6.72 to 49.3&nbsp;AU.{{Efn|1=Calculated using the [[semi-major axis]] and the [[orbital eccentricity]].
<br />
28×(1-0.76) = 6.72 AU.
<br />
28×(1+0.76) = 49.28 AU.}} Previous studies found a lower eccentricity {{Nowrap|1=(e = {{val|0.23|0.11|0.23}})}}, as well as a lower [[semi-major axis]] of {{val|17.6|4.3|8.1|u=AU}}. This difference is explained due to the increased orbit coverage and astrometric acceleration used in the new value.<ref name=Franson2022/> HD 984 B is expected to be in [[periastron]] in 2028.<ref name=Franson2022/>

===Physical parameters===
Its mass was firstly estimated using the age of the system by Meshkat ''et al.'' (2015) and Jonson-Groh ''et al.'' (2017).<ref name=Johnson-Groh2017/><ref name=Meshkat2015/> Using this method, masses ranging from {{Jupiter mass|34|link=y}} to {{Jupiter mass|94|link=}} are obtained, assuming ages of 30 and 200 million years respectively.<ref name=Johnson-Groh2017/> Assuming the largest mass and age, HD 984 B would be in the stellar mass regime. A 2022 study found a dynamical mass of {{Val|61|4|u=Jupiter mass}}, which places HD 984 B in the brown dwarf regime at 99.7% confidence. This mass was obtained using [[Astrometry|astrometric]] acceleration, new [[direct imaging]] of the object and [[radial velocity]] measurements, and is consistent with [[Evolution|evolutionary]] models.<ref name=Franson2022/>

HD 984 B is a hot brown dwarf<ref name=Costes2024/> 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 {{val|2730|ul=K}}.<ref name=Johnson-Groh2017/> At a luminosity of about {{val|1.318|e=-3|ul=solar luminosity}},<ref name=Franson2022/> this corresponds to a radius of {{Solar radius calculator|type=logLT|-2.88|2730|decimals=3|unit=y|link=y}} ({{Jupiter radius|1.576|link=y}}) based on the [[Stefan–Boltzmann law]].{{efn|name=radius}} Its [[metallicity]], i.e. its abundance of elements other than hydrogen and helium, is {{Val|-0.62|0.02|u=[[Common logarithm|dex]]}}, just one-fourth of the [[Sun|solar]] metalicity (0 dex), although this value might be biased. This brown dwarf more likely formed via [[gravitational collapse]] or [[disk instability]] rather than [[Core-accretion theory|core accretion]].<ref name=Costes2024/>

==See also==
* [[Epsilon Indi]], [[54 Piscium]] and [[Eta Telescopii]], another stars orbited by brown dwarfs
* [[List of brown dwarfs]]
* [[List of stars in Cetus]]

== Notes ==
<references group="lower-alpha" />
<references group="note" />

== References ==
{{Reflist|refs=

<ref name=Franson2022>{{Cite journal |last1=Franson |first1=Kyle |last2=Bowler |first2=Brendan P. |last3=Brandt |first3=Timothy D. |last4=Dupuy |first4=Trent J. |last5=Tran |first5=Quang H. |last6=Brandt |first6=G. Mirek |last7=Li |first7=Yiting |last8=Kraus |first8=Adam L. |date=2022-02-01 |title=Dynamical Mass of the Young Substellar Companion HD 984 B |journal=The Astronomical Journal |volume=163 |issue=2 |pages=50 |doi=10.3847/1538-3881/ac35e8 |doi-access=free |arxiv=2111.01803 |bibcode=2022AJ....163...50F |issn=0004-6256}}</ref>

<ref name=Johnson-Groh2017>{{Cite journal |last1=Johnson-Groh |first1=Mara |last2=Marois |first2=Christian |last3=De Rosa |first3=Robert J. |last4=Nielsen |first4=Eric L. |last5=Rameau |first5=Julien |last6=Blunt |first6=Sarah |last7=Vargas |first7=Jeffrey |last8=Ammons |first8=S. Mark |last9=Bailey |first9=Vanessa P. |last10=Barman |first10=Travis S. |last11=Bulger |first11=Joanna |last12=Chilcote |first12=Jeffrey K. |last13=Cotten |first13=Tara |last14=Doyon |first14=René |last15=Duchêne |first15=Gaspard |date=2017-04-01 |title=Integral Field Spectroscopy of the Low-mass Companion HD 984 B with the Gemini Planet Imager |journal=The Astronomical Journal |volume=153 |issue=4 |pages=190 |doi=10.3847/1538-3881/aa6480 |doi-access=free |arxiv=1703.02607 |bibcode=2017AJ....153..190J |issn=0004-6256}}</ref>

<ref name=binoculars>{{cite web |last1=Macrobert |first1=Alan |title=Binoculars: Halfway to a Telescope |url=https://skyandtelescope.org/astronomy-resources/binoculars-halfway-to-a-telescope/ |website=Sky & Telescope |date=28 July 2006 |publisher=American Astronomical Society |access-date=June 16, 2024}}</ref> 

<ref name=Britannica>{{Cite web |date=2024-04-22 |title=Brown dwarf {{!}} Astronomy, Formation & Characteristics {{!}} Britannica |url=https://www.britannica.com/science/brown-dwarf |access-date=2024-04-24 |website=www.britannica.com |language=en}}</ref>

<ref name=Costes2024>{{Cite journal |last1=Costes |first1=J. C. |last2=Xuan |first2=J. W. |last3=Vigan |first3=A. |last4=Wang |first4=J. |last5=D’Orazi |first5=V. |last6=Mollière |first6=P. |last7=Baker |first7=A. |last8=Bartos |first8=R. |last9=Blake |first9=G. A. |last10=Calvin |first10=B. |last11=Cetre |first11=S. |last12=Delorme |first12=J. |last13=Doppmann |first13=G. |last14=Echeveri |first14=D. |last15=Finnerty |first15=L. |date=2024-06-01 |title=Fresh view of the hot brown dwarf HD 984 B through high-resolution spectroscopy |arxiv=2404.11523 |journal=[[Astronomy & Astrophysics]] |language=en |volume=686 |pages=A294 |doi=10.1051/0004-6361/202348370 |bibcode=2024A&A...686A.294C |issn=0004-6361}}</ref>

<ref name=SIMBAD>{{Cite simbad|HD+984|title=HD 984}}</ref>

<ref name=GaiaDR3>{{Cite Gaia EDR3|2431157720981843200}}</ref>

<ref name=Nielsen2020>{{Cite journal |last1=Nielsen |first1=E. L. |last2=De Rosa |first2=R. |last3=Macintosh |first3=B. |last4=Wang |first4=J. |last5=Ruffio |first5=J. |last6=Chiang |first6=E. |last7=Marley |first7=M. |last8=Saumon |first8=D. |last9=Savransky |first9=D. |last10=Gemini Planet Imager Exoplanet Survey Team |date=2020-01-01 |title=The Gemini Planet Imager Exoplanet Survey: Giant Planet and Brown Dwarf Demographics from 10-100 AU |journal=American Astronomical Society Meeting Abstracts #235 |url=https://ui.adsabs.harvard.edu/abs/2020AAS...23528002N |volume=235 |pages=280.02|bibcode=2020AAS...23528002N }}</ref>

<ref name=Coordinates>{{Cite web |title=coord 00 14 10.2528328224-07 11 56.812684848 (ICRS, J2000, 2000.0), radius: 7 arcmin |url=https://simbad.cds.unistra.fr/simbad/sim-coo?Coord=00+14+10.2528328224-07+11+56.812684848&CooFrame=ICRS&CooEqui=2000.0&CooEpoch=J2000&Radius.unit=arcmin&submit=Query+around&Radius=7 |access-date=2024-06-15 |website=simbad.cds.unistra.fr}}</ref> 

<ref name=Meshkat2015>{{Cite journal |last1=Meshkat |first1=T. |last2=Bonnefoy |first2=M. |last3=Mamajek |first3=E. E. |last4=Quanz |first4=S. P. |last5=Chauvin |first5=G. |last6=Kenworthy |first6=M. A. |last7=Rameau |first7=J. |last8=Meyer |first8=M. R. |last9=Lagrange |first9=A.-M. |last10=Lannier |first10=J. |last11=Delorme |first11=P. |date=2015-11-01 |title=Discovery of a Low-Mass Companion to the F7V star HD 984 |journal=Monthly Notices of the Royal Astronomical Society |volume=453 |issue=3 |pages=2379–2387 |doi=10.1093/mnras/stv1732 |doi-access=free |arxiv=1507.08291 |issn=0035-8711}}</ref>

<ref name="Stellar Catalog">{{Cite web |title=★ HD 984 |url=https://www.stellarcatalog.com/stars/hd-984 |access-date=2024-06-16 |website=Stellar Catalog |language=en}}</ref>

<ref name=Bowler2023>{{Cite journal |last1=Bowler |first1=Brendan P. |last2=Tran |first2=Quang H. |last3=Zhang |first3=Zhoujian |last4=Morgan |first4=Marvin |last5=Ashok |first5=Katelyn B. |last6=Blunt |first6=Sarah |last7=Bryan |first7=Marta L. |last8=Evans |first8=Analis E. |last9=Franson |first9=Kyle |last10=Huber |first10=Daniel |last11=Nagpal |first11=Vighnesh |last12=Wu |first12=Ya-Lin |last13=Zhou |first13=Yifan |date=2023-04-01 |title=Rotation Periods, Inclinations, and Obliquities of Cool Stars Hosting Directly Imaged Substellar Companions: Spin–Orbit Misalignments Are Common |journal=The Astronomical Journal |volume=165 |issue=4 |pages=164 |doi=10.3847/1538-3881/acbd34 |doi-access=free |arxiv=2301.04692 |bibcode=2023AJ....165..164B |issn=0004-6256}}</ref>

<ref name=csiro>{{citation |title=The Colour of Stars |date=December 21, 2004 |work=Australia Telescope, Outreach and Education |publisher=[[Commonwealth Scientific and Industrial Research Organisation]] |url=http://outreach.atnf.csiro.au/education/senior/astrophysics/photometry_colour.html |access-date=2012-01-16 |url-status=dead |archive-url=https://web.archive.org/web/20120318151427/http://outreach.atnf.csiro.au/education/senior/astrophysics/photometry_colour.html |archive-date=2012-03-18 }}</ref>

}}

{{Sky|00|14|10.253|-|07|11|56.81}}
{{Cetus}}

[[Category:F-type main-sequence stars]]
[[Category:Cetus]]
[[Category:Brown dwarfs]]
[[Category:TIC objects]]
[[Category:Henry Draper Catalogue objects|000984]]
[[Category:2MASS objects]]
[[Category:Hipparcos objects|001134]]