{{Short description|Star in the constellation Draco}} {{Starbox begin}} <!--{{Starbox image | image = [[Image:Barnardstar2006.jpg|250px]] | caption = The location of Barnard's Star }}--> {{Starbox observe | epoch = [[J2000.0]] | constell = [[Draco (constellation)|Draco]] | pronounce = | ra = {{RA|16|40|57.16}}<ref name="simbad">{{cite web | title=SIMBAD Query Result: GD 356 | publisher=[[Centre de Données astronomiques de Strasbourg]] | work=SIMBAD | url=http://simbad.u-strasbg.fr/simbad/sim-basic?Ident=GD+356&submit=SIMBAD+search | access-date=13 June 2012}}</ref>{{failed verification|date=August 2025}} | dec = {{DEC|+53|41|09.6}}<ref name="simbad" />{{failed verification|date=August 2025}} | appmag_v = 15.06<ref name="simbad" />{{failed verification|date=August 2025}} }} {{Starbox character | type = [[white dwarf]]<ref name="Walters2021" /> | class = DAHe<ref name="Walters2021" /> | appmag_1_passband = B | appmag_1 = ~15.39<ref name="simbad" /> | appmag_3_passband = R | appmag_3 = ~15.1<ref name="simbad" /> | appmag_4_passband = I | appmag_4 = ~14.0<ref name="simbad" /> | appmag_5_passband = J | appmag_5 = ~14.493<ref name="simbad" /> | appmag_6_passband = H | appmag_6 = ~14.479<ref name="simbad" /> | appmag_7_passband = K | appmag_7 = ~14.369<ref name="simbad" /> | r-i = <!--R-I color--> | v-r = <!--V-R color--> | b-v = +0.33<ref name="simbad" />{{failed verification|date=August 2025}} | u-b = −0.52<ref name="simbad"/>{{failed verification|date=August 2025}} | variable = 0.2% over 115 minutes }} {{Starbox astrometry | radial_v = 25 | prop_mo_ra = {{val|-119.425|0.031 }} | prop_mo_dec = {{val|-190.438|0.031}} | pm_footnote =<ref name="Gaia EDR3"/> | parallax = 49.6501 | p_error = 0.0207 | parallax_footnote =<ref name="Gaia EDR3"/> | absmag_v = 13.43{{cn|date=August 2025}} }} {{Starbox detail | source = <!--[source url]--> | mass = 0.67 | radius = | gravity = 8<!--Surface gravity (given as the base 10 logarithm expressed in cgs units)--> | luminosity_visual = | luminosity_bolometric = | temperature = 7510 | metal = | rotation = 115 minutes | age_gyr = About 2.1 }} {{Starbox catalog | names = [[Gliese-Jahreiss catalogue|Gliese]] 1205, [[Luyten-Palomar proper motion catalogue|LP]] 137-43, EGGR 329, WD 1639+537 }} {{Starbox reference | Simbad = GD+356 | ARICNS = 02194 }} {{Starbox end }} '''GD 356''' is a [[white dwarf]] in the constellation of [[Draco (constellation)|Draco]] showing an unusual emission of circular polarised light. The star is 65 light years from earth.<ref name="Muir">{{cite journal|last=Muir|first=Hazel|date=1 August 1998|title=The Earth could be in for an electrifying time|journal=New Scientist|issue=2145|page=7}}</ref>
Other catalog names for this are '''LP 137-43''', '''EGGR 329''' and '''WD 1639+537'''.<ref name="Ferrario"/>
==Spectral peculiarities== The class of this white dwarf is DA<sub>e</sub> meaning that it has a cool helium rich atmosphere.<ref name="Wickramasinghe"/> This star exhibits emission lines showing the [[Zeeman effect]] in the hydrogen [[Balmer lines|Balmer]] spectrum.<ref name="Wickramasinghe">{{cite journal |last1=Wickramasinghe|first1=Dayal T. |first2=Jay|last2=Farihi |first3=Christopher A.|last3=Tout |first4=Lilia|last4=Ferrario|author4-link=Lilia Ferrario |first5=Richard J.|last5=Stancliffe |date=9 February 2010|title=Does GD356 have a Terrestrial Planetary Companion?|journal=[[Monthly Notices of the Royal Astronomical Society]]|volume=404|issue=4|doi=10.1111/j.1365-2966.2010.16417.x|pages=1984–1991|doi-access=free |arxiv=1002.1761|bibcode = 2010MNRAS.404.1984W |s2cid=119255099 }}</ref> GD 356 belongs to a class of [[high field magnetic white dwarf]]s (HFMWD), but it is unique in that the split lines are purely emission lines with no absorption. The emission region appears to be due to a heated upper layer in the [[photosphere]] in which the magnetic field is uniform to within 10%.<ref name="Wickramasinghe"/> The emission can be produced by an atmosphere at 7500K in a gravity field of 10<sup>6</sup> ms<sup>−2</sup> and a magnetic field of 13 megaGauss. The magnetically split emission lines, H<sub>α</sub> and H<sub>β</sub>, are circularly polarised.<ref name="Ferrario">{{cite journal |last1=Ferrario|first1=Lilia|author1-link=Lilia Ferrario |first2=Dayal T.|last2=Wickramasinghe |first3=James|last3=Liebert |first4=Gary D.|last4=Schmidt |first5=John H.|last5=Bieging |date=1997|title=The Magnetic Field and Emission-Line Spectrum of the Remarkable White Dwarf GD 356|journal=Monthly Notices of the Royal Astronomical Society|volume=289|issue=1|pages=105–116|bibcode = 1997MNRAS.289..105F|doi=10.1093/mnras/289.1.105|doi-access=free}}</ref> One explanation is that it is caused by a large electric current flowing between the poles of the star and a highly conducting planet.<ref name="Muir"/> This planet was not detected in a later, more detailed analysis with new data. Rejecting the idea of an orbiting planet.<ref name="Walters2021"/> Other explanations such as being due to [[Bondi-Hoyle accretion]] or due to a corona are ruled out by the lack of radio and X-ray emissions. Accretion of gas at a low rate over a broad area of the star, only results in heating at levels high in the atmosphere and not down to the opacity depth of 1.0 as observed with these lines.
The spectrum does not vary over periods of hours or days. This indicates that the rotation axis must match the magnetic dipole axis. The power radiated by the emission lines is 10<sup>27</sup> erg s<sup>−1</sup>. Overall light from the white dwarf varies by 0.2% smoothly over a period of 117 minutes.<ref name="Wickramasinghe"/> Explanations given for the variation are a dark spot rotating with the star. This could be near the rotation pole when viewed nearly edge on, or could be on the equator with the pole pointing roughly towards Earth.<ref name="Brinkworth">{{cite journal|last=Brinkworth|first=C. S.|author2=M. R. Burleigh |author3=G. A. Wynn |author4=T. R. Marsh |date=2004|title=Photometric variability of the unique magnetic white dwarf GD 356|journal=Monthly Notices of the Royal Astronomical Society|volume=384|issue=3|pages=L33–L37|doi=10.1111/j.1365-2966.2004.07538.x|doi-access=free |arxiv = astro-ph/0312311 |bibcode = 2004MNRAS.348L..33B |s2cid=15677179}}</ref>
==Properties== The mass of GD 356 is {{Solar mass|0.67}} whereas when it was a main sequence star it would have had a mass of {{Solar mass|3.25}}. In order to reach a temperature of 7510 K it would have become a white dwarf about 1.6 Gya. Prior to this the main sequence lifetime would have been 500 million years giving it a total age of 2.1 billion years.<ref name="Wickramasinghe"/> The current magnitude is 15.<ref name="Greenstein"/>
The absolute visual magnitude is +13.43±0.16. Proper motion is 0.24" pa, in direction 212°.<ref name="Ferrario"/> The trigonometric parallax is 21.1 parsecs. Tangential motion is 25 km<sup>−1</sup>.<ref name="Greenstein"/>
==Spectrum== The H<sub>α</sub> line splitting is 44.5 nm. In similar white dwarfs an absorption line is expected to be seen instead, so that means the emission has sufficient energy to overpower any absorption.<ref name="Greenstein">{{cite journal|last=Greenstein|first=Jesse L.|author2=James K. McCarthy|date=15 February 1985|title=Emission lines in the magnetic white dwarf GD 356|journal=Astrophysical Journal, Part 1|volume=289|pages=732–747|issn=0004-637X|bibcode = 1985ApJ...289..732G |doi = 10.1086/162937 }}</ref> The emission was originally discovered by Jesse L. Greenstein.<ref name="Greenstein"/> The original H<sub>α</sub> line has a wavelength at 655.2 nm and is called the π component. The blue shifted component σ<sup>−</sup> has wavelength 633.4 nm and red shifted component line σ<sup>+</sup> is at 678.2 nm.<ref name="Greenstein"/>
==Rejected companion== The unipolar-inductor theory says that a high-conduction companion orbits. As it moves through the star's magnetic field, a high voltage is produced between the star facing side of the planet and the dark side. A current then flows along field lines to the point on the star where the field lines meet the star's photosphere, the current is completed through the photosphere heating it up.<ref name="Wickramasinghe"/>
A planet in a close orbit would develop the shape of the Roche potential and is very likely to be molten due to tidal heating.<ref name="Wickramasinghe"/> A planet with a density of over five g/cm<sup>3</sup> is stable at an orbital period longer than 4.7 hours. A planet in this kind of orbit may have a temperature of 560 K and could be detectable in infrared if it was large enough.<ref name="Wickramasinghe"/>
Infrared observations rule out a large companion such as a brown dwarf or other large planet over twelve Jupiter masses. This is based on the expected temperature of 2.1 billion year old planets.<ref name="Wickramasinghe"/>
A planet could possibly get into this situation by evaporating while orbiting inside the gaseous shell of the [[red giant]] and at the same time having its [[orbit decay]] due to [[bow-shock friction]] with the gas. Tides induce on the expanded star by the planet would also cause the orbit to decay, rather than expand as might have been expected to loss of gas from the star. These possibilities have been studied because that is the expected [[future of the Earth]]. Another hypothesis is that close-in planets could have formed during the merger of two white dwarfs.<ref name="Wickramasinghe"/>
The white dwarf was studied in detail in 2021, rejecting the idea of such a companion. An orbiting body should produce additional signals, such as changes in the photometric period. These additional signals are not detected and the researchers find multiple potential points of failure in the unipolar inductor model. The researchers conclude that the [[Chromosphere|chromospheric]] emission is [[Intrinsic and extrinsic properties|intrinsic]] and not due to a companion.<ref name="Walters2021" />
== DAHe prototype and explanation == The unusual spectroscopic features leads to the spectroscopic classification of DAHe (A=hydrogen, H=magnetic field without polarization, e=emission lines) for this white dwarf.<ref name="Walters2021" /> Several other DAHe that are similar were found, making GD 356 the prototype of this spectral type.<ref name="Reding2023"/><ref name="Manser2023"/> After the rejection of the unipolar inductor model for GD 356 another explanation was necessary.<ref name="Walters2021" /> These DAHe systems all cluster closely in the [[Gaia (spacecraft)|Gaia]] [[Hertzsprung–Russell diagram|Hertzsprung-Russell diagram]] (HR diagram). Therefore it was suspected that DAHe white dwarfs have strong magnetic fields due to a crystallization-driven convective dynamo. The cause for the emergence of emission lines was unclear at the time.<ref name="Manser2023" /> It is suggested that DAHe white dwarfs are experiencing [[carbon]]–[[oxygen]] crystallization and distillation of [[Isotopes of neon|<sup>22</sup>Neon]] inside these white dwarfs. The presence of distilled <sup>22</sup>Ne acts as a dynamo for a strong magnetic field that heats the atmosphere near the magnetic poles via [[Joule heating|ohmic heating]]. The equator experiences almost no heating. Misalignment between rotational axis and magnetic axis will cause the chromospheric emission line spots to rotate in and out of view, resulting in the observed variability. It is thought that the DAHe white dwarfs are the result of mergers between two objects, of which one is a carbon-oxygen core white dwarf and the other is a helium-core white dwarf or a [[subgiant]]. This is evident from the large amount of required <sup>22</sup>Ne, the high mass and the fast rotation of DAHe white dwarfs.<ref name="Lanza2024"/> More research is however needed, as there are two DAe white dwarfs without a detected magnetic field. They are thought to be similar to DAHe white dwarfs, as they cluster closely with DAHe in the Gaia HR diagram.<ref name="Elms2023"/>
==References== {{Reflist |refs=
<ref name="Walters2021">{{Cite journal |last1=Walters |first1=N. |last2=Farihi |first2=J. |last3=Marsh |first3=T. R. |last4=Bagnulo |first4=S. |last5=Landstreet |first5=J. D. |last6=Hermes |first6=J. J. |last7=Achilleos |first7=N. |last8=Wallach |first8=A. |last9=Hart |first9=M. |last10=Manser |first10=C. J. |date=2021-05-01 |title=A test of the planet-star unipolar inductor for magnetic white dwarfs |journal=Monthly Notices of the Royal Astronomical Society |volume=503 |issue=3 |pages=3743–3758 |arxiv=2103.01993 |bibcode=2021MNRAS.503.3743W |doi=10.1093/mnras/stab617 |doi-access=free |issn=0035-8711}}</ref>
<ref name="Reding2023">{{Cite journal |last1=Reding |first1=Joshua S. |last2=Hermes |first2=J. J. |last3=Clemens |first3=J. C. |last4=Hegedus |first4=R. J. |last5=Kaiser |first5=B. C. |date=2023-06-01 |title=Two new white dwarfs with variable magnetic Balmer emission lines |journal=Monthly Notices of the Royal Astronomical Society |volume=522 |issue=1 |pages=693–699 |arxiv=2302.10207 |bibcode=2023MNRAS.522..693R |doi=10.1093/mnras/stad760 |doi-access=free |issn=0035-8711}}</ref>
<ref name="Manser2023">{{Cite journal |last1=Manser |first1=Christopher J. |last2=Gänsicke |first2=Boris T. |last3=Inight |first3=Keith |last4=Robert |first4=Akshay |last5=Ahlen |first5=S. |last6=Allende Prieto |first6=C. |last7=Brooks |first7=D. |last8=Cooper |first8=A. P. |last9=de la Macorra |first9=A. |last10=Font-Ribera |first10=A. |last11=Honscheid |first11=K. |last12=Kisner |first12=T. |last13=Landriau |first13=M. |last14=Meisner |first14=Aaron M. |last15=Miquel |first15=R. |date=2023-06-01 |title=DAHe white dwarfs from the DESI Survey |journal=Monthly Notices of the Royal Astronomical Society |volume=521 |issue=4 |pages=4976–4994 |arxiv=2302.01335 |bibcode=2023MNRAS.521.4976M |doi=10.1093/mnras/stad727 |doi-access=free |issn=0035-8711}}</ref>
<ref name="Lanza2024">{{Cite journal |last1=Lanza |first1=A. F. |last2=Rui |first2=N. Z. |last3=Farihi |first3=J. |last4=Landstreet |first4=J. D. |last5=Bagnulo |first5=S. |date=2024-09-01 |title=Atmospheric heating and magnetism driven by 22Ne distillation in isolated white dwarfs |url=https://ui.adsabs.harvard.edu/abs/2024A&A...689A.233L/abstract |journal=Astronomy and Astrophysics |volume=689 |pages=A233 |arxiv=2407.19289 |bibcode=2024A&A...689A.233L |doi=10.1051/0004-6361/202449947 |issn=0004-6361}}</ref>
<ref name="Elms2023">{{Cite journal |last1=Elms |first1=Abbigail K. |last2=Tremblay |first2=Pier-Emmanuel |last3=Gänsicke |first3=Boris T. |last4=Swan |first4=Andrew |last5=Melis |first5=Carl |last6=Bédard |first6=Antoine |last7=Manser |first7=Christopher J. |last8=Munday |first8=James |last9=Hermes |first9=J. J. |last10=Dennihy |first10=Erik |last11=Nitta |first11=Atsuko |last12=Zuckerman |first12=Ben |date=2023-10-01 |title=An emerging and enigmatic spectral class of isolated DAe white dwarfs |journal=Monthly Notices of the Royal Astronomical Society |volume=524 |issue=4 |pages=4996–5015 |arxiv=2307.09186 |bibcode=2023MNRAS.524.4996E |doi=10.1093/mnras/stad2171 |doi-access=free |issn=0035-8711}}</ref>
<ref name="Gaia EDR3">{{Cite Gaia EDR3|1425909733315616000}}</ref> }}
{{Stars of Draco}}
[[Category:Draco (constellation)]] [[Category:White dwarfs]] [[Category:Emission-line stars]] [[Category:Gliese and GJ objects|1205]]