WR 21a

{{Short description|Star in the constellation Carina}}

{{Starbox begin

| name=WR 21a

}}

{{Starbox image

| image=

{{Location mark

|image=Westerlund 2.jpg|alt=|float=center|width=280

|label=|position=right

|mark=Green circle.png|mark_width=10|mark_link=WR 21a

|x=77|y=589

}}

|caption=Location of WR 21a (circled) near RCW 49

|credit=ESO

}}

{{Starbox observe

| epoch=J2000

| ra= {{RA|10|25|56.502}}

| dec= {{DEC|−57|48|43.52}}

| appmag_v=12.661{{cite journal|bibcode=2013AJ....145...44Z|title=The Fourth US Naval Observatory CCD Astrograph Catalog (UCAC4)|journal=The Astronomical Journal|volume=145|issue=2|pages=44|last1=Zacharias|first1=N.|last2=Finch|first2=C. T.|last3=Girard|first3=T. M.|last4=Henden|first4=A.|last5=Bartlett|first5=J. L.|author5-link=Jennifer Bartlett (astronomer)|last6=Monet|first6=D. G.|last7=Zacharias|first7=M. I.|year=2013|doi=10.1088/0004-6256/145/2/44|arxiv = 1212.6182 |s2cid=119299381}}

| constell=Carina

}}

{{Starbox character

| class=O3/WN5ha + O3Vz((f*)){{cite journal|bibcode=2016MNRAS.455.1275T|arxiv=1510.03609|title=The mass of the very massive binary WR21a|journal=Monthly Notices of the Royal Astronomical Society|volume=455|issue=2|pages=1275–1281|last1=Tramper|first1=F.|last2=Sana|first2=H.|last3=Fitzsimons|first3=N. E.|last4=De Koter|first4=A.|last5=Kaper|first5=L.|last6=Mahy|first6=L.|last7=Moffat|first7=A.|year=2016|doi=10.1093/mnras/stv2373|doi-access=free |s2cid=44364798}}

| b-v=+1.507

| u-b=

| variable=Eclipsing

}}

{{Starbox astrometry

| radial_v=

| prop_mo_ra=−4.079

| prop_mo_dec=2.617

| parallax=0.2497

| p_error=0.0143

| parallax_footnote = {{cite Gaia DR3|5351703390282380800}}

| dist_ly=

| dist_pc=

| absmag_v = −7.20

}}

{{Starbox orbit

| reference = {{cite journal |bibcode=2022MNRAS.516.1149B |title=The winking eye of a very massive star: WR 21a revealed as an eclipsing binary by TESS |last1=Barbá |first1=Rodolfo H. |last2=Gamen |first2=Roberto C. |last3=Martín-Ravelo |first3=Pablo |last4=Arias |first4=Julia I. |last5=Morrell |first5=Nidia I. |journal=Monthly Notices of the Royal Astronomical Society |year=2022 |volume=516 |issue=1 |page=1149 |doi=10.1093/mnras/stac2173 |doi-access=free |arxiv=2109.06311 }}

| primary = WR

| name = O

| period =

| period_unitless = {{val|31.67855|0.00002}} days

| axis =

| axis_unitless = {{solar radius|230}}{{cite journal|bibcode=2016A&A...590A.113G|title=The X-ray light curve of the massive colliding wind Wolf-Rayet + O binary WR21a|volume=1604|pages=A113|arxiv=1604.01536|last1=Gosset|first1=Eric|last2=Naze|first2=Yael|year=2016|doi=10.1051/0004-6361/201527051|journal=Astronomy & Astrophysics|s2cid=119183652}}

| eccentricity = {{val|0.695|0.007}}

| inclination = {{val|61.8|1.5}}

| node =

| periastron =

| periarg =

| periarg_primary =

| k1 = {{val|158.0|2.7}}

| k2 = {{val|278.1|2.8}}

}}

{{Starbox detail

| source=

| component1=WR

| mass=94.4

| radius={{val|23.3|1.6}}

| luminosity=1,510,000

| temperature=45,000{{cite journal|bibcode=2005A&A...440..743B|arxiv=astro-ph/0506363|title=Multiwavelength studies of WR 21a and its surroundings|journal=Astronomy and Astrophysics|volume=440|issue=2|pages=743–750|last1=Benaglia|first1=P.|last2=Romero|first2=G. E.|last3=Koribalski|first3=B.|last4=Pollock|first4=A. M. T.|year=2005|doi=10.1051/0004-6361:20042617|s2cid=17048378}}

| gravity=3.69

| metal=

| rotation=

| age=

| component2=O

| mass2=53.6

| radius2={{val|14.8|2.0}}

| luminosity2=1,050,000

| temperature2=50,680

| gravity2=3.81

| metal2=

| rotation2=

| age_myr2=1.5

}}

{{Starbox catalog

| names= WR 21a, 2MASS J10255650-5748435

}}

{{Starbox reference

|Simbad = WR+21a

}}

{{Starbox end}}

WR 21a is an eclipsing binary star in the constellation Carina. It includes one of the most massive known stars and is one of the most massive binaries.

WR 21a lies near the Westerlund 2 open cluster{{cite journal|bibcode=2015MNRAS.446.3797H|arxiv=1411.0879|title=Reddening, distance, and stellar content of the young open cluster Westerlund 2|journal=Monthly Notices of the Royal Astronomical Society|volume=446|issue=4|pages=3797–3819|last1=Hur|first1=Hyeonoh|last2=Park|first2=Byeong-Gon|last3=Sung|first3=Hwankyung|last4=Bessell|first4=Michael S.|last5=Lim|first5=Beomdu|last6=Chun|first6=Moo-Young|last7=Sohn|first7=Sangmo Tony|year=2015|doi=10.1093/mnras/stu2329|doi-access=free |s2cid=119221719}} and likely to be an ejected member.{{citation|arxiv=2009.05773|year=2020|title=The Villafranca catalog of Galactic OB groups|doi=10.1051/0004-6361/202038228 |last1=Maíz Apellániz |first1=J. |last2=Crespo Bellido |first2=P. |last3=Barbá |first3=R. H. |last4=Fernández Aranda |first4=R. |last5=Sota |first5=A. |journal=Astronomy & Astrophysics |volume=643 |pages=A138 |s2cid=221655188 }}

The distance of WR 21a was not definitively known until Gaia mission. There have been estimates from 2.85 kpc to around 8 kpc, with consequent uncertainties in the system luminosity. The larger distance was preferred because of consistency with the derived orbital parameters.{{cite journal|bibcode=2013A&A...555A..50C|arxiv=1305.4309|title=The distance to the young open cluster Westerlund 2|journal=Astronomy & Astrophysics|volume=555|pages=A50|last1=Carraro|first1=G.|last2=Turner|first2=D.|last3=Majaess|first3=D.|last4=Baume|first4=G.|year=2013|doi=10.1051/0004-6361/201321421|s2cid=53756444}}

Every 31 days and 16 hours the two stars in this system revolve around each other. The inclination of the orbit means that only very shallow eclipses are observed and the brightness dips by only about 0.05 magnitudes. There are also even smaller brightness variations attributed to the heartbeat effect where the closest passage of the stars in their eccentric orbits creates brightness changes as the two stars illuminate each other. There may also be tidally-excited oscillations producing further small variations.

The colliding winds of the two stars produce extremely high temperatures and luminous x-ray emission. The system is also bright at radio wavelengths.