J/A+A/649/A167 Hubble spectroscopy of LB-1 (LS V +22 25) (Lennon+, 2021)
Hubble spectroscopy of LB-1:
Comparison with B+black-hole and Be+stripped-star models.
Lennon D.J., Maiz Apelliniz J., Irrgang A., Bohlin R., Deustua S.,
Dufton P.L., Simon-Diaz S., Herrero A., Casares J., Munoz-Darias T.,
Smartt S.J., Gonzalez Hernandez J.I., de Burgos A.
<Astron. Astrophys. 649, A167 (2021)>
=2021A&A...649A.167L 2021A&A...649A.167L (SIMBAD/NED BibCode)
ADC_Keywords: Binaries, spectroscopic ; Stars, B-type ; Spectrophotometry ;
Spectra, ultraviolet
Keywords: techniques: spectroscopic - binaries: spectroscopic -
stars: black holes - stars: early-type - stars: evolution -
stars: fundamental parameters
Abstract:
LB-1 has variously been proposed as either an X-ray dim B-type star
plus black hole (B+BH) binary, or a Be star plus an inflated stripped
star (Be+Bstr) binary. The Space Telescope Imaging Spectrograph (STIS)
on board HST was used to obtain a flux-calibrated spectrum that is
compared with non-LTE spectral energy distributions (SED) and line
profiles for the proposed models. The Hubble data, together with the
Gaia EDR3 parallax, provide tight constraints on the properties and
stellar luminosities of the system. In the case of the Be+Bstr model
we adopt the published flux ratio for the Be and Bstr stars,
re-determine the Teff of the Bstr using the silicon ionization
balance, and infer Teff for the Be star from the fit to the SED. We
derive stellar parameters consistent with previous results, but with
greater precision enabled by the Hubble SED. While the Be+Bstr model
is a better fit to the HeI lines and cores of the Balmer lines in the
optical, the B+BH model provides a better fit to the SiIV resonance
lines in the UV. The analysis also implies that the Bstr star has
roughly twice solar silicon abundance, which is difficult to reconcile
with a stripped star origin. The Be star on the other hand has a
rather low luminosity, and a spectroscopic mass inconsistent with its
possible dynamical mass. The fit to the UV can be significantly
improved by reducing the Teff and radius of the Be star, though at the
expense of leading to a different mass ratio. In the B+BH model, the
single B-type spectrum is a good match to the UV spectrum. Adopting a
mass ratio of 5.1±0.1 (Liu et al., 2020ApJ...900...42L 2020ApJ...900...42L) implies a BH
mass of 21+9-8 solar masses.
Description:
Low resolution wide slit spectrophotometry in the UV optical were
obtained using the Space Telescope Imaging Spectrograph (STIS), and
near-IR grism spectra obtained with the Wide Field Camera 3 (WFC3).
STIS data were processed using custom IDL programs that enable a
superior flux calibration compared to the standard pipeline products.
This processing is identical to the methods for Hubble CALSPEC flux
standards (Bohlin et al., 2019AJ....158..211B 2019AJ....158..211B). Small offsets of the
wavelength scale of approximately 1 pixel, due to slight off-centering
in the wide slit, were corrected using the predicted radial velocity
of the narrow-lined star using the ephemeris of the system. The
following (vacuum) merge points were used in splicing together the
G140L, G230L, G430L and G750L data; 1678, 3065, and 5450 Angstroems.
The near-IR data from WFC3 using the G102 and G141 grisms are spliced
into the spectrum at 9575 Angstroems.
Objects:
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RA (2000) DE Designation(s)
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06 11 49.08 +22 49 32.7 LB-1 = LS V +22 25
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File Summary:
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FileName Lrecl Records Explanations
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ReadMe 80 . This file
table2.dat 72 3875 Spectrophotometric data
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Byte-by-byte Description of file: table2.dat
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Bytes Format Units Label Explanations
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1- 12 F12.6 0.1nm lambda Vacuum wavelength (in Å)
13- 24 E12.4 10mW/m2/nm Flux Flux (in erg/cm2/s/Å)
25- 36 E12.4 10mW/m2/nm e_Flux Statistical error (in erg/cm2/s/Å)
37- 48 E12.4 10mW/m2/nm s_Flux Systematic error (in erg/cm2/s/Å)
49- 58 F10.3 0.1nm FWHM Full-width-half-maximum of resolution
element
59- 62 I4 --- Quality [0/1] Quality flag (1=good, 0=bad)
63- 72 F10.1 s Totexp Total exposure time
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Acknowledgements:
Danny J. Lennon, dlennon(at)iac.es
References:
Goudfrooij et al., 2006PASP..118.1455G 2006PASP..118.1455G
Bohlin et al., 2014PASP..126..711B 2014PASP..126..711B
Bohlin et al., 2019AJ....158..211B 2019AJ....158..211B
(End) Danny J. Lennon [IAC, Spain], Patricia Vannier [CDS] 30-Mar-2021