Revealing the elusive companion of the red giant binary 2MASSJ05215658+4359220 from UV HST and Astrosat-UVIT data

Luciana Bianchi, John Hutchings, Ralph Bohlin, David Thilker, Emanuele Berti
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Abstract

Black hole demographics in different environments is critical in view of recent results on massive-stars binarity, and of the multi-messenger detectability of compact objects mergers. But the identification and characterization of non-interacting black holes is elusive, especially in the sparse field stellar population. A candidate non-interactive black hole (BH)+red giant (RG) binary system, 2MASSJ05215658+4359220, was identified by Thompson et al.(2019). We obtained Astrosat/UVIT Far-Ultraviolet (FUV) imaging and Hubble Space Telescope (HST) UV-optical imaging and spectroscopy of the source, to test possible scenarios for the optically-elusive companion. HST/STIS spectra from about 1,600 to 10,230Ang are best fit by the combination of two stellar sources, a red giant with Teff=4250 (uncertainty 150K), logg=2.0, Radius_RG=27.8Rsun (assuming a single-temperature atmosphere), and a subgiant companion with Teff=6,000K, Radius_comp=2.7Rsun, or Teff=5,270K, Radius_comp=4.2Rsun using models with one-tenth or one-third solar metallicity respectively, logg=3.0, extinction E(B-V)=0.50(uncertainty 0.2), adopting the DR3 Gaia distance D=2,463pc (uncertainty 120pc). No FUV data existed prior to our programs. STIS spectra give an upper limit of 10e-17ergs cm-2 s-1 Ang-1 shortward of 2300Ang; an upper limit of >25.7ABmag was obtained in two UVIT FUV broad-bands. The non-detection of FUV flux rules out a compact companion such as a hot WD. The STIS spectrum shows strong MgII lambda2800Ang emission, typical of chromospherically active red giants. The masses inferred by comparison with evolutionary tracks, about 1 Msun for the red giant and between 1.1 and 1.6Msun for the subgiant companion, suggest past mass transfer, although the red giant currently does not fill its Roche lobe. WFC3 imaging in F218W, F275W, F336W, F475W, and F606W shows an unresolved source in all filters.
从紫外 HST 和 Astrosat-UVIT 数据中揭示红巨星双星 2MASSJ05215658+4359220 难以捉摸的伴星
鉴于近期关于大质量恒星二元性的研究结果,以及紧凑天体合并的多信使可探测性,不同环境下的黑洞人口统计至关重要。但是对非相互作用黑洞的识别和描述却很难,尤其是在稀疏场恒星群中。汤普森等人(2019)发现了一个候选的非相互作用黑洞(BH)+红巨星(RG)双星系统--2MASSJ05215658+4359220。我们获得了该星源的Astrosat/UVIT远紫外(FUV)成像和哈勃太空望远镜(HST)紫外-光学成像和光谱,以检验光学上易消失的伴星的可能情况。HST/STIS光谱从大约1,600Ang到10,230Ang与两个恒星源的组合最为拟合,其中一个红巨星的Teff=4250(不确定性为150K),logg=2.0, Radius_RG=27.8Rsun (假设为单温大气), 以及亚巨星伴星,Teff=6,000K, Radius_comp=2.7Rsun, 或 Teff=5,270K, Radius_comp=4.2Rsun,分别使用十分之一或三分之一太阳金属性的模型,logg=3.0,消光 E(B-V)=0.50(不确定性 0.2),采用DR3 Gaia 距离 D=2463pc(不确定性 120pc)。在我们的计划之前,没有任何 FUV 数据。STIS 光谱给出了 2300Ang 以北 10e-17ergs cm-2 s-1 Ang-1 的上限;在两个 UVIT FUV 宽波段中得到了大于 25.7ABmag 的上限。由于没有探测到 FUV 通量,因此排除了热 WD 等紧凑伴星的可能性。STIS 光谱显示出强烈的 MgII lambda2800Ang 发射,这是典型的色球活动红巨星。通过与演化轨迹比较推断出的质量(红巨星约为1Msun,亚巨星伴星在1.1-1.6Msun之间)表明,尽管红巨星目前并没有填满它的罗切叶,但过去发生过质量转移。WFC3在F218W、F275W、F336W、F475W和F606W的成像在所有滤光片中都显示出一个未分辨源。
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