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{{Short description|Wolf Rayet star in the constellation Carina}}
] created by a powerful ] impacting material expelled during earlier stages of the star's life (ESA/Hubble & NASA Acknowledgement: Judy Schmidt)]]
{{Use dmy dates|date=July 2017}}
{{Starbox begin
| name = WR 31a
}}
{{Starbox image
| image = ]
| caption = WR 31a is surrounded by a blue bubble nebula created by a powerful ] impacting material expelled during earlier stages of the star's life.
| credit = ESA/Hubble & NASA; Acknowledgement: Judy Schmidt
}}
{{Starbox observe
| epoch = J2000.0
| constell = ]
| ra = {{RA|10|53|59.5777}}<ref name=dr3/>
| dec = {{DEC|−60|26|44.361}}<ref name=dr3/>
| appmag_v = 10.85<ref name=tycho2/>
}}
{{Starbox character
| type = ]
| class = WN11h<ref name=toala/>
| appmag_1_passband = B
| appmag_1 = {{val|11.64|0.10}}
| appmag_2_passband = V
| appmag_2 = {{val|10.85|0.08}}
| appmag_3_passband = R
| appmag_3 = {{val|10.56|0.02}}
| appmag_4_passband = G
| appmag_4 = {{val|10.033|0.004}}
| appmag_5_passband = J
| appmag_5 = {{val|7.323|0.024}}
| appmag_6_passband = H
| appmag_6 = {{val|6.72|0.05}}
| appmag_7_passband = K
| appmag_7 = {{val|6.097|0.021}}
| appmag_8_passband = g
| appmag_8 = {{val|11.53|0.32}}
| r-i = 0.5
| v-r = 0.29
| b-v = 0.79
| j-h = 0.603
| j-k = 1.226
| variable = ]<ref name=naze/><ref name=smith2015/>
}}
{{Starbox astrometry
| radial_v =
| prop_mo_ra = −5.890<ref name=dr3/>
| prop_mo_dec = 2.842<ref name=dr3/>
| parallax = 0.1152
| p_error = 0.0131
| parallax_footnote=<ref name=dr3/>
| dist_ly =
| dist_pc =
| absmag_v = −6.71<ref name=wr/>
}}
{{Starbox detail
| source =
| mass = {{val|17|7|4}}<ref name=smith/>
| radius = {{val|29.8|11.9|6.1}}<ref name=smith/>
| luminosity = {{nowrap|1,820,000<ref name=vangenderen/>}}
| temperature = {{nowrap|30,200 (1985–1992)<ref name=vangenderen/>}} {{nowrap|27,500 (1991)<ref name=smith/>}}
| metallicity_fe =
| gravity =
| age =
}}
{{Starbox catalog
| names = {{nowrap|] 31a,}} {{nowrap|] 3-519,}} {{nowrap|He 3-519,<ref name=smith/>}} {{nowrap|] J10535958-6026444,}} {{nowrap|] DR1 5338229111491506304,}} {{nowrap|] 08958-01166,}} {{nowrap|] S1113013697,}} {{nowrap|] 5308747,}} {{nowrap|] 148-066854,}} {{nowrap|] 10520-6010,}} {{nowrap|] 8928-1166-1,}} {{nowrap|] 1050–59,}} {{nowrap|] G288.9347-00.8046}}
}}
{{Starbox reference
| Simbad = WR_31a
}}
{{Starbox end}}


'''WR 31a''', commonly referred to as '''Hen 3-519''', is a ] (WR) star in the southern constellation of ] that is surrounded by an expanding ]. It is not a classical old stripped-envelope WR star, but a young massive star which still has some hydrogen left in its atmosphere.
'''WR 31a''' is a ] in the southern constellation of ] that is surrounded by an expanding optical blue bubble nebula<ref>{{cite web|url=https://www.nasa.gov/image-feature/goddard/2016/hubbles-blue-bubble|title=Hubble's Blue Bubble|work=NASA|accessdate=1 March 2016}}</ref> Some media sources have wrongly claimed this deep-sky object was recently discovered by the ],<ref>{{cite web|url=http://www.inquisitr.com/2841239/hubble-captures-blue-bubble-star-20-times-bigger-a-million-times-brighter-than-sun/|title=Hubble Captures "Blue Bubble' Star - 20 Times Bigger, a Million Times Brighter than the Sun's|work=Inquisitr|accessdate=2 March 2016}}</ref> but this nebula was originally found by ] in 1953 and designated as the ] '''Hf 39'''.<ref name="VizieR">{{cite web|url=http://vizier.u-strasbg.fr/viz-bin/VizieR-S?PN%20G288.9-00.8|title=VizieR|work=CDS|accessdate=2 March 2016}}</ref> Other planetary nebula catalogue names included, He 3-319, ESO 128-18, and Wray 15-682.<ref name="VizieR"/>


== History of observations ==
Since 2013, the object has been considered not a planetary nebula but rather, an expanding gas shell, formally classified as a ] or WR nebula.<ref name=frew>{{cite journal|doi=10.1093/mnras/sts393|title=A catalogue of integrated Hα fluxes for 1258 Galactic planetary nebulae|journal=Monthly Notices of the Royal Astronomical Society|volume=431|pages=2|year=2013|last1=Frew|first1=D. J.|last2=Bojicic|first2=I. S.|last3=Parker|first3=Q. A.|bibcode=2013MNRAS.431....2F}}</ref> Its observed expansion velocity is 365&nbsp;km.s<sup>−1</sup>, estimated to be some 2.4 parsecs across.<ref name=wise>{{Cite journal|arxiv=1503.06878v1|last1= Toalá|first1= J. A.|title= WISE morphological study of Wolf-Rayet nebulae|journal= Astronomy & Astrophysics|volume= 578|pages= A66|last2= Guerrero|first2= M. A.|last3= Ramos-Larios|first3= G.|last4= Guzmán|first4= V.|year= 2015|doi= 10.1051/0004-6361/201525706}}</ref>
WR 31a was first published in 1952 as one of six peculiar ] stars, but not given a designation at that time. It was described as having numerous ] lines with unusually broad emission components.<ref name=henize>{{cite journal |bibcode=1952ApJ...115..133H |title=Six Peculiar Hα-Emission Stars |journal=Astrophysical Journal |volume=115 |pages=133 |last1=Henize |first1=Karl G. |year=1952 |doi=10.1086/145522|doi-access=free }}</ref> A year later it was listed as being associated with nebulosity, at the time thought to be a planetary nebula.<ref name=hoffleit/>


In 1976, it was included in a catalogue of southern emission-line stars, the third catalogue of emission objects compiled by ]. The designation {{nowrap|Hen 3-519}}, sometimes {{nowrap|He 3-519}}, was adopted as the most common identifier for this star.<ref name=hen>{{cite journal |bibcode=1976ApJS...30..491H |title=Observations of southern emission-line stars |journal=Astrophysical Journal Supplement Series |volume=30 |pages=491 |last1=Henize |first1=K. G. |year=1976 |doi=10.1086/190369|doi-access=free }}</ref> In 2001, it was listed in The Seventh Catalogue of Galactic Wolf-Rayet stars as entry 31a,<ref name=wr/> but it is still frequently called {{nowrap|Hen 3-519}}.<ref name=smith2015 /><ref name=naze />
Its central highly luminous star (TYC 8928-1166-1) is invisible to the naked-eye, shining at 10.85V magnitude,<ref>{{cite web|url=http://simbad.u-strasbg.fr/simbad/sim-id?Ident=WR+31a&NbIdent=1&Radius=2&Radius.unit=arcmin&submit=submit+id|title=SIMBAD|work=CDS|accessdate=2 March 2016}}</ref> whose spectral type is WN11h.<ref name="wise" /> Mass of the central star is estimated to be at least 20 times that of the Sun, and it will likely become a ] type II event in the future.<ref>{{cite web|url=https://www.spacetelescope.org/images/potw1608a/|title=Blue bubble in Carina|work=NASA|accessdate=2 March 2016}}</ref> Distance is estimated to be about 9,200 parsecs or 30,000 light-years.


In 1994, WR 31a was first described as a candidate ] (cLBV) after a detailed ] study with the ].<ref name=smith />
==References==
{{Reflist}}


Measurements of its ], published in 2017 as a result of the '']'' mission, suggested a much closer (6,500 ]; 2,000 ]) distance to {{nowrap|WR 31a}} as well as to its neighbor, the luminous blue variable ]. It was thought that, if this distance was confirmed by ''Gaia''{{'s}} second data release (DR2) in 2018, this would mean both stars were much less luminous than previously thought, and both may instead be former ] stars.<ref name=SmithN2017>{{cite journal |title=The Canonical Luminous Blue Variable AG Car and Its Neighbor Hen 3-519 are Much Closer than Previously Assumed |journal=The Astronomical Journal |first1=Nathan |last1=Smith |first2=Keivan G. |last2=Stassun |volume=153 |issue=3 |at=125 |date=March 2017 |doi=10.3847/1538-3881/aa5d0c |bibcode=2017AJ....153..125S |arxiv=1610.06522|s2cid=119296636 |doi-access=free }}</ref> However, ] returned a parallax of {{val|0.0418|0.0299|ul=mas}}; using ], Smith ''et al.'' (2018) calculated its distance to be {{val|31,200|8,500|6,000|fmt=commas|ul=ly}} ({{val|9,570|2,600|1,850|fmt=commas|ul=pc}}).<ref name=NSmith2018>{{cite journal |title=On the Gaia DR2 distances for Galactic Luminous Blue Variables |arxiv=1805.03298 |first1=Nathan |last1=Smith |first2=Mojgan |last2=Aghakhanloo |first3=Jeremiah W. |last3=Murphy |first4=Keivan G. |last4=Stassun |first5=Maria R. |last5=Drout |first6=Jose H. |last6=Groh |journal=Monthly Notices of the Royal Astronomical Society |year=2019 |volume=488 |issue=2 |pages=1760–1778 |display-authors=1 |doi=10.1093/mnras/stz1712 |doi-access=free |bibcode=2019MNRAS.488.1760S |s2cid=119267371 }}</ref> ] lists a parallax of {{val|0.1152|0.0156|u=mas}}, again indicating a large distance.<ref name=dr3/> A statistical analysis leads to a distance of {{val|7,315|993|935|ul=pc|fmt=commas}}.<ref name=bailer-jones/>

== Blue bubble ==
WR 31a is surrounded by a ] nearly {{convert|8|ly|pc|spell=in}} wide. The ] (HST) has captured a striking image of the nebula, rendering it as a thin blue bubble. In this false colour image, the blue colours represents red visible light (605&nbsp;nm), while orange-red colours represent near ] radiation at 814&nbsp;nm.<ref>{{cite web |url=https://www.nasa.gov/image-feature/goddard/2016/hubbles-blue-bubble |title=Hubble's Blue Bubble |work=NASA |access-date=1 March 2016|date=2016-02-26 }}</ref> Some media sources have wrongly claimed this deep-sky object was recently discovered by the HST,<ref>{{cite web |url=http://www.inquisitr.com/2841239/hubble-captures-blue-bubble-star-20-times-bigger-a-million-times-brighter-than-sun/ |title=Hubble Captures "Blue Bubble' Star—20 Times Bigger, a Million Times Brighter than the Sun's |work=Inquisitr |date=29 February 2016 |access-date=2 March 2016}}</ref> but this nebula was originally found by ] in 1953 and designated as the ] '''Hf 39'''.<ref name=hoffleit>{{cite journal |bibcode=1953AnHar.119...37H |title=A preliminary survey of nebulosities and associated B-stars in Carina |journal=Annals of Harvard College Observatory |volume=119 |pages=37 |last1=Hoffleit |first1=Dorrit |year=1953}}</ref> Other planetary nebula catalogue names include ESO 128-18 and Wray 15-682.<ref name=hoffleit />

Since 2013, the nebula has been considered not as a planetary nebula, but rather as a much larger expanding gas shell, formally classified as a ] or WR nebula.<ref name=frew>{{cite journal |doi=10.1093/mnras/sts393 |title=A catalogue of integrated Hα fluxes for 1258 Galactic planetary nebulae |journal=Monthly Notices of the Royal Astronomical Society |volume=431 |issue=1 |pages=2–26 |year=2013 |last1=Frew |first1=D. J. |last2=Bojicic |first2=I. S. |last3=Parker |first3=Q. A. |doi-access=free |bibcode=2013MNRAS.431....2F |arxiv=1211.2505|s2cid=119174103 }}</ref> Its observed expansion velocity is {{convert|365|km/s|mi/s|abbr=on}}, and is estimated to be some {{convert|2.4|pc|ly}} across.<ref name=wise>{{Cite journal |arxiv=1503.06878 |last1=Toalá |first1=J. A. |title=WISE morphological study of Wolf-Rayet nebulae |journal=Astronomy & Astrophysics |volume=578 |pages=A66 |last2=Guerrero |first2=M. A. |last3=Ramos-Larios |first3=G. |last4=Guzmán |first4=V. |year=2015 |doi=10.1051/0004-6361/201525706 |bibcode=2015A&A...578A..66T|s2cid=55776698 }}</ref> The dynamical age of the nebula is estimated at 18,000 years and the total mass of ionised gas at {{solar mass|2–3}}.<ref name=smith />

== Properties ==
WR 31a itself, the highly luminous central star of the nebula, is invisible to the naked-eye at magnitude 10.85V. It has a spectral classification of WN11h, indicating a WR star with strong N{{smallcaps|ii}} emission but no N{{smallcaps|iii}} emission, and with hydrogen features visible in the spectrum.<ref name=wise/> The spectral classification WN11 was created for this star and ] since they did not fit into any existing spectral type and appeared to constitute an extension of the WR nitrogen sequence to cooler temperatures.<ref name=smith>{{cite journal |bibcode=1994A&A...281..833S |title=A study of the luminous blue variable candidate He 3-519 and its surrounding nebula |journal=Astronomy and Astrophysics |volume=281 |issue=3 |pages=833–854 |last1=Smith |first1=L. J. |last2=Crowther |first2=P. A. |last3=Prinja |first3=R. K. |year=1994}}</ref> The progenitor's mass of the central star is estimated to have been about 45 times that of the Sun,<ref name=smith2015>{{cite journal |bibcode=2015MNRAS.447..598S |arxiv=1406.7431 |title=Luminous blue variables are antisocial: Their isolation implies that they are kicked mass gainers in binary evolution |journal=Monthly Notices of the Royal Astronomical Society |volume=447 |issue=1 |pages=598–617 |last1=Smith |first1=Nathan |last2=Tombleson |first2=Ryan |year=2015 |doi=10.1093/mnras/stu2430|doi-access=free |s2cid=119284620 }}</ref> and this massive star will likely explode as a ] in the future.<ref name="Blue bubble in Carina">{{cite web |url=https://www.spacetelescope.org/images/potw1608a/ |title=Blue bubble in Carina |work=NASA |access-date=2 March 2016}}</ref>

The spectrum of WR 31a shows ]s, most strongly on the dominant H{{smallcaps|i}}, He{{smallcaps|i}}, and N{{smallcaps|ii}} lines. These profiles may show dramatic changes on a timescale of weeks, with the absorption components of the lines sometimes disappearing completely.<ref name=smith/>

== Variability ==
] ] for WR 31a, plotted from ] data<ref name="ASAS-SN"/>]]
No significant brightness changes have been detected in WR 31a, but it has been listed as a candidate luminous blue variable because of its luminosity, temperature, and mass loss properties.<ref name=smith /> Van Genderen catalogued it as a dormant or ex-LBV because of the lack of characteristic LBV variations.<ref name=vangenderen>{{cite journal |bibcode=2001A&A...366..508V |title=S Doradus variables in the Galaxy and the Magellanic Clouds |journal=Astronomy and Astrophysics |volume=366 |issue=2 |pages=508–531 |last1=Van Genderen |first1=A. M. |year=2001 |doi=10.1051/0004-6361:20000022|doi-access=free }}</ref> Possible small brightness changes have been seen in ] (ASAS) data,<ref name=asas>{{cite journal |bibcode=2002AcA....52..397P |arxiv=astro-ph/0210283 |title=The All Sky Automated Survey. Catalog of Variable Stars. I. 0 h – 6 h ''Quarter'' of the Southern Hemisphere |journal=Acta Astronomica |volume=52 |pages=397–427 |last1=Pojmanski |first1=G. |year=2002}}</ref> WR 31a is included as a variable star in the International Variable Star Index,<ref name=vsx>{{cite web |url=https://www.aavso.org/vsx/index.php?view=detail.top&revid=256789 |title=Hen 3-519 |publisher=] |work=International Variable Star Index |access-date=30 September 2016 |date=4 January 2010}}</ref> although not in the ].<ref name=gcvs>{{cite journal |bibcode=2009yCat....102025S |title=VizieR Online Data Catalog: General Catalogue of Variable Stars (Samus+ 2007–2013) |journal=VizieR On-line Data Catalog: B/GCVS. Originally Published in: 2009yCat....102025S |volume=1 |display-authors=etal |last1=Samus |first1=N. N. |last2=Durlevich |first2=O. V. |year=2009}}</ref>

== References ==
{{Reflist|refs=

<ref name=tycho2>{{cite book |bibcode=2000A&A...355L..27H |title=The Tycho-2 catalogue of the 2.5&nbsp;million brightest stars |journal=Astronomy and Astrophysics |volume=355 |pages=L27 |last1=Høg |first1=E. |last2=Fabricius |first2=C. |last3=Makarov |first3=V. V. |last4=Urban |first4=S. |last5=Corbin |first5=T. |last6=Wycoff |first6=G. |last7=Bastian |first7=U. |last8=Schwekendiek |first8=P. |last9=Wicenec |first9=A. |year=2000 |doi=10.1888/0333750888/2862|isbn=978-0333750889 }}</ref>

<ref name="ASAS-SN">{{cite web |url=https://asas-sn.osu.edu/variables/lookup |website=ASAS-SN Variable Stars Database|title=ASAS-SN Variable Stars Database|publisher=ASAS-SN |access-date=25 September 2024}}</ref>

<ref name=naze>{{cite journal |bibcode=2012A&A...538A..47N |arxiv=1111.6375 |title=The first X-ray survey of Galactic luminous blue variables |journal=Astronomy & Astrophysics |volume=538 |pages=A47 |last1=Nazé |first1=Y. |last2=Rauw |first2=G. |last3=Hutsemékers |first3=D. |year=2012 |doi=10.1051/0004-6361/201118040|s2cid=43688343 }}</ref>

<!-- <ref name=dr1>{{cite journal |bibcode=2016yCat.1337....0G |title=VizieR Online Data Catalog: Gaia DR1 |journal=Astronomy and Astrophysics |author=Gaia Collaboration |date=2016}}</ref> -->

<ref name=toala>{{cite journal |bibcode=2015A&A...578A..66T |arxiv=1503.06878 |title=WISE morphological study of Wolf–Rayet nebulae |journal=Astronomy & Astrophysics |volume=578 |pages=A66 |last1=Toalá |first1=J. A. |last2=Guerrero |first2=M. A. |last3=Ramos-Larios |first3=G. |last4=Guzmán |first4=V. |year=2015 |doi=10.1051/0004-6361/201525706|s2cid=55776698 }}</ref>

<ref name=wr>{{cite journal |bibcode=2001NewAR..45..135V |title=The VIIth catalogue of galactic Wolf–Rayet stars |journal=New Astronomy Reviews |volume=45 |issue=3 |pages=135–232 |last1=Van Der Hucht |first1=Karel A. |author-link1=Karel van der Hucht|year=2001 |doi=10.1016/S1387-6473(00)00112-3}}</ref>

<!-- <ref name=gaia>{{cite journal |bibcode=2016A&A...595A...2G |title=Gaia Data Release 1. Summary of the astrometric, photometric, and survey properties |journal=Astronomy and Astrophysics |last1=Brown |first1=A. G. A. |last2=Vallenari |first2=A. |last3=Prusti |first3=T. |last4=de Bruijne |first4=J. |last5=Mignard |first5=F. |display-authors=etal |date=2016 |arxiv=1609.04172 |doi=10.1051/0004-6361/201629512 |volume=595 |pages=A2}}</ref> -->

<ref name=bailer-jones>{{cite journal |bibcode=2021AJ....161..147B |title=Estimating Distances from Parallaxes. V. Geometric and Photogeometric Distances to 1.47 Billion Stars in Gaia Early Data Release 3 |last1=Bailer-Jones |first1=C. A. L. |last2=Rybizki |first2=J. |last3=Fouesneau |first3=M. |last4=Demleitner |first4=M. |last5=Andrae |first5=R. |journal=The Astronomical Journal |year=2021 |volume=161 |issue=3 |page=147 |doi=10.3847/1538-3881/abd806 |arxiv=2012.05220 |s2cid=228063812 |doi-access=free }}</ref>

<ref name=dr3>{{cite Gaia DR3|5338229115839425664}}</ref>

}} <!-- end of reflist -->

{{Stars of Carina}}

{{DEFAULTSORT:WR 31a}}
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] ]
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Latest revision as of 12:01, 25 September 2024

Wolf Rayet star in the constellation Carina

WR 31a

WR 31a is surrounded by a blue bubble nebula created by a powerful stellar wind impacting material expelled during earlier stages of the star's life.
Credit: ESA/Hubble & NASA; Acknowledgement: Judy Schmidt
Observation data
Epoch J2000.0      Equinox J2000.0
Constellation Carina
Right ascension 10 53 59.5777
Declination −60° 26′ 44.361″
Apparent magnitude (V) 10.85
Characteristics
Evolutionary stage Wolf–Rayet
Spectral type WN11h
Apparent magnitude (B) 11.64±0.10
Apparent magnitude (V) 10.85±0.08
Apparent magnitude (R) 10.56±0.02
Apparent magnitude (G) 10.033±0.004
Apparent magnitude (J) 7.323±0.024
Apparent magnitude (H) 6.72±0.05
Apparent magnitude (K) 6.097±0.021
Apparent magnitude (g) 11.53±0.32
B−V color index 0.79
V−R color index 0.29
R−I color index 0.5
J−H color index 0.603
J−K color index 1.226
Variable type cLBV
Astrometry
Proper motion (μ) RA: −5.890 mas/yr
Dec.: 2.842 mas/yr
Parallax (π)0.1152 ± 0.0131 mas
Distanceapprox. 28,000 ly
(approx. 8,700 pc)
Absolute magnitude (MV)−6.71
Details
Mass17+7
−4 M☉
Radius29.8+11.9
−6.1 R☉
Luminosity1,820,000 L☉
Temperature30,200 (1985–1992) 27,500 (1991) K
Other designations
WR 31a, Hen 3-519, He 3-519, 2MASS J10535958-6026444, Gaia DR1 5338229111491506304, GSC 08958-01166, GSC2 S1113013697, UCAC2 5308747, UCAC4 148-066854, IRAS 10520-6010, TYC 8928-1166-1, AAVSO 1050–59, MSX5C G288.9347-00.8046
Database references
SIMBADdata

WR 31a, commonly referred to as Hen 3-519, is a Wolf–Rayet (WR) star in the southern constellation of Carina that is surrounded by an expanding Wolf–Rayet nebula. It is not a classical old stripped-envelope WR star, but a young massive star which still has some hydrogen left in its atmosphere.

History of observations

WR 31a was first published in 1952 as one of six peculiar emission line stars, but not given a designation at that time. It was described as having numerous P Cygni type lines with unusually broad emission components. A year later it was listed as being associated with nebulosity, at the time thought to be a planetary nebula.

In 1976, it was included in a catalogue of southern emission-line stars, the third catalogue of emission objects compiled by Karl Gordon Henize. The designation Hen 3-519, sometimes He 3-519, was adopted as the most common identifier for this star. In 2001, it was listed in The Seventh Catalogue of Galactic Wolf-Rayet stars as entry 31a, but it is still frequently called Hen 3-519.

In 1994, WR 31a was first described as a candidate luminous blue variable (cLBV) after a detailed spectrographic study with the Anglo-Australian Telescope.

Measurements of its parallax, published in 2017 as a result of the Gaia mission, suggested a much closer (6,500 light-year; 2,000 parsec) distance to WR 31a as well as to its neighbor, the luminous blue variable AG Carinae. It was thought that, if this distance was confirmed by Gaia's second data release (DR2) in 2018, this would mean both stars were much less luminous than previously thought, and both may instead be former red supergiant stars. However, Gaia Data Release 2 returned a parallax of 0.0418±0.0299 mas; using Bayesian inference, Smith et al. (2018) calculated its distance to be 31,200+8,500
−6,000 ly (9,570+2,600
−1,850 pc). Gaia Data Release 3 lists a parallax of 0.1152±0.0156 mas, again indicating a large distance. A statistical analysis leads to a distance of 7,315+993
−935 pc.

Blue bubble

WR 31a is surrounded by a shell of ionised gas nearly eight light-years (2.5 pc) wide. The Hubble Space Telescope (HST) has captured a striking image of the nebula, rendering it as a thin blue bubble. In this false colour image, the blue colours represents red visible light (605 nm), while orange-red colours represent near infrared radiation at 814 nm. Some media sources have wrongly claimed this deep-sky object was recently discovered by the HST, but this nebula was originally found by Ellen Dorrit Hoffleit in 1953 and designated as the planetary nebula Hf 39. Other planetary nebula catalogue names include ESO 128-18 and Wray 15-682.

Since 2013, the nebula has been considered not as a planetary nebula, but rather as a much larger expanding gas shell, formally classified as a Wolf–Rayet nebula or WR nebula. Its observed expansion velocity is 365 km/s (227 mi/s), and is estimated to be some 2.4 parsecs (7.8 ly) across. The dynamical age of the nebula is estimated at 18,000 years and the total mass of ionised gas at 2–3 M☉.

Properties

WR 31a itself, the highly luminous central star of the nebula, is invisible to the naked-eye at magnitude 10.85V. It has a spectral classification of WN11h, indicating a WR star with strong Nii emission but no Niii emission, and with hydrogen features visible in the spectrum. The spectral classification WN11 was created for this star and AG Carinae since they did not fit into any existing spectral type and appeared to constitute an extension of the WR nitrogen sequence to cooler temperatures. The progenitor's mass of the central star is estimated to have been about 45 times that of the Sun, and this massive star will likely explode as a supernova in the future.

The spectrum of WR 31a shows P Cygni profiles, most strongly on the dominant Hi, Hei, and Nii lines. These profiles may show dramatic changes on a timescale of weeks, with the absorption components of the lines sometimes disappearing completely.

Variability

A visual band light curve for WR 31a, plotted from ASAS-SN data

No significant brightness changes have been detected in WR 31a, but it has been listed as a candidate luminous blue variable because of its luminosity, temperature, and mass loss properties. Van Genderen catalogued it as a dormant or ex-LBV because of the lack of characteristic LBV variations. Possible small brightness changes have been seen in All Sky Automated Survey (ASAS) data, WR 31a is included as a variable star in the International Variable Star Index, although not in the General Catalogue of Variable Stars.

References

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  10. ^ Hoffleit, Dorrit (1953). "A preliminary survey of nebulosities and associated B-stars in Carina". Annals of Harvard College Observatory. 119: 37. Bibcode:1953AnHar.119...37H.
  11. Henize, K. G. (1976). "Observations of southern emission-line stars". Astrophysical Journal Supplement Series. 30: 491. Bibcode:1976ApJS...30..491H. doi:10.1086/190369.
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  13. Smith, Nathan; et al. (2019). "On the Gaia DR2 distances for Galactic Luminous Blue Variables". Monthly Notices of the Royal Astronomical Society. 488 (2): 1760–1778. arXiv:1805.03298. Bibcode:2019MNRAS.488.1760S. doi:10.1093/mnras/stz1712. S2CID 119267371.
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