A new candidate Luminous Blue Variable
Donald F. Figer, Francisco Najarro, Maria Messineo, J. Simon Clark, Karl M. Menten
aa r X i v : . [ a s t r o - ph . S R ] S e p D raft version S eptember
24, 2020
Preprint typeset using L A TEX style emulateapj v. 12 / / A NEW CANDIDATE LUMINOUS BLUE VARIABLE D onald F. F iger , F rancisco N ajarro , M aria M essineo , J. S imon C lark , K arl M. M enten Draft version September 24, 2020
ABSTRACTWe identify IRAS 16115 − ∼ > . L ⊙ ), ensuring supergiant status,has a temperature similar to LBVs, is photometrically and spectroscopically variable, and is surrounded bywarm dust. Its near-infrared spectrum shows the presence of several lines of H I, He I, Fe II, Fe [II], Mg II,and Na I with shapes ranging from pure absorption and P Cygni profiles to full emission. These characteristicsare often observed together in the relatively rare LBV class of stars, of which only ≈
20 are known in theGalaxy. The key to the new classification is the fact that we compute a new distance and extinction that yieldsa luminosity significantly in excess of those for post-AGB PPNe, for which the initial masses are < M ⊙ .Assuming single star evolution, we estimate an initial mass of ≈ M ⊙ . Subject headings:
Luminous blue variable stars (944) — Stellar evolution (1599) — Massive stars (732) —Supergiant stars (1661) — Infrared sources (793) — Stellar mass loss (1613) INTRODUCTION
Luminous blue variables (LBVs) have a distinct set of ob-served characteristics (Conti 1984). They have luminositiesof supergiants, temperatures above 10 kK, non-periodic vari-ability, and evidence of eruptions vis a vis circumstellar ejecta(e.g. Clark et al. 2005). They are inferred to be post-mainsequence descendants of massive stars (Meynet et al. 2011).Their spectrophotometric variations are on the order of 1–2mag over timescales of years at roughly constant bolometricluminosity. During giant eruptions, their brightness changesby up to 3 mag (e.g. van Genderen et al. 1997). They areoften surrounded by ionized gas and warm dust, both ev-idence of past eruptions (Clark et al. 2005). The archety-pal example, LBV η Car, is surrounded by the Homuncu-lus, material inferred to have been ejected by the star duringthe great eruption starting in 1837 (Smith & Owocki 2006).These eruptions carry with them an extraordinary amount ofmaterial, such as the ≈ M ⊙ of material surrounding the η Car (Smith & Ferland 2007). The e ff ective mass-loss ratesduring these eruptions is extraordinarily high, such as up to ∼ M ⊙ yr − for η Car during its great eruption. Their evo-lutionary paths in the Hertzsprung-Russel diagram are uncer-tain, due in part to poor statistics, but they are clearly near,or sometimes even above, the Humphreys-Davidson limit(Humphreys & Davidson 1994). There are approximately 20known LBVs in the Galaxy and a similar number of candi-dates (Smith et al. 2019). LBVs may be progenitors of atleast some supernovae (c.f., Smith et al. 2011; Burgasser et al.2012; Dessart et al. 2015), and Allan et al. (2020) argue thatan LBV directly collapsed to a black hole. fi[email protected] Center for Detectors, Rochester Institute of Technology, 54 MemorialDrive, Rochester, NY 14623, USA Centro de Astrobiologa (CSIC / INTA), ctra. de Ajalvir km. 4, 28850Torrejn de Ardoz, Madrid, Spain Key Laboratory for Researches in Galaxies and Cosmology, Univer-sity of Science and Technology of China, Chinese Academy of Sciences,Hefei, Anhui, 230026, China Department of Physics and Astronomy, The Open University, WaltonHall, Milton Keynes, MK7 6AA, UK Max-Planck-Institut f¨ur Radioastronomie, Auf dem H¨ugel 69, D-53121 Bonn, Germany
In this paper, we demonstrate that IRAS 16115 − M ⊙ . We argue that it is more appropriately identified as anLBV, similar to how He 3-519 and the Pistol star were re-classified from PPNe to LBVs (Figer et al. 1998). In this Let-ter, we review available observations, present newly-reducedspectra, and reinterpret the nature of this star. In Sect. 2,we present photometry from the literature and newly-reducedspectroscopy. In Sect. 3, we estimate the distance. In Section4, we describe and characterize the dusty nebula. In Section5, we estimate the extinction. In Section 6, we estimate thestellar luminosity. In Section 7, we compare the star to otherLBVs. DATA
Spectra from the Literature
Su´arez et al. (2006) obtained an optical spectrum of thestar in June 1990 using the 1.5 m ESO telescope. Theywere searching for planetary nebulae (PNe), and categorizedIRAS 16115 − &
21, given that it has G = ∼
7, and such an object would be a challenge to ob-serve with a 1.5 m telescope.Oudmaijer et al. (1995) published a K-band spectrum withrelatively low signal-to-noise ratio (S / N) of the object. Itappeared featureless in the narrow wavelength range theycovered (2.2–2.4 µ m), except perhaps for a Mg II line near2.40 µ m. Several years later, Weldrake et al. (2003) observedIRAS 16115 − − .They identified emission lines in the hydrogen Paschen andBracket series, as well as from Fe II and 2.089 µ m, Mg II at2.138 and 2.144 µ m, and Na I at 2.206 and 2.209 µ m. Figer et al. New Spectra
Figure 1 shows two observed spectra (solid-black), cor-rected for telluric absorption and emission, obtained withSINFONI on the ESO Very Large Telescope (VLT), togetherwith a model fit (dashed red line, see Sect.6). The spec-trum in the upper panel was observed in 2013 with a 0 . ′′ . ′′
025 plate scale(Program ID: 097.D-0033(A)) . The spectra were reduced us-ing the ESO pipeline with wavelength calibration set by OHlines for the 2013 data set. These lines were not visible inthe 2016 data, so we used arc lamp lines in data taken dur-ing the daytime. After the wavelength calibration, we mea-sured the centroids of the observed emission lines and esti-mate the S / N which varies from ∼
70 in the telluric pollutedregions to ∼ µ m. The spectra dis-play strong Br- γ emission, with a weak P Cyg absorption dipand the corresponding He I hydrogenic components in absorp-tion. H I lines in the Pfund series up to Pf are clearly seen inemission longward of 2.3 µ m, with a P Cygni shape presentin higher-resolution and S / N spectrum from 2016 which de-creases from Pf till Pf . The 2.112 / µ m He I doubletlines are in absorption, and the 2.058 µ m He I line is in emis-sion with a potential P Cygni profile. This line is blended withthe Fe II 2.060 µ m. Two Mg II doublets in emission are seennear 2.14 and 2.41 µ m. Fe II and Fe [II] emission lines areseen throughout the spectrum. The Na I doublet is detectednear 2.21 µ m. Many lines are present in similar strengths inboth spectra. The iron and helium lines are clearly strongerin the 2013 spectrum (upper panel). The spectrum looks sim-ilar to that in Weldrake et al. (2003), although the Fe II linestrengths in the 2016 spectrum provide a best match.The spectra are nearly identical to those for qF362(Najarro et al. 2009) and G79.29 + + + > <
13 kK, which taken togethersuggests a spectral type of B5-8 at the time of the observation(see Figure 8 in Messineo et al. 2011). The widths from theFe II 1.974 and 2.089 µ m emission lines which are formed inthe mid-outer wind and hence provide a reliable estimate of v ∞ are ≈
350 km s − , suggesting a wind speed ( ∼
175 km s − )that is typical for LBVs (Smith 2014).IRAS 16115 − γ line is 172 Å ± ± ± ff erence between the two measurements andthe mean. The equivalent widths for some of the iron linesvary by up to 400%. The shape of the blend of the apparentBr γ P Cygni absorption line and the He I photospheric linealso appears to change between the two observations.
Photometry
The data in Table 1 were taken at many di ff erent times andwith very di ff erent beam widths. The circumstellar emissionis clearly included for some of them, e.g., in the IRAS andAkari data.IRAS 16115 − µ m, 185 Jy at 25 µ m,500 Jy at 60 µ m, and 255 Jy at 100 µ m. The IRAS colors are[12]-[25] = = / Habing diagram (Figure 5b in van der Veen & Habing1988), the [60]-[100] color falls in region V, though the [12]-[25] is redder than 2 mag. Region V is the region of PNe andnon-variable stars with cold envelopes.
Photometric variations
The source was analyzed for photometric variability withdata from the Di ff use Infrared Background Experiment(DIRBE) instrument on the Cosmic Background Explorer(COBE) at 1.25 µ m, 2.2 µ m, 3.5 µ m, and 4.9 µ m and re-ported as a non-variable star (Price et al. 2010). The flux den-sity increased over the 3.6 year time period of the observa-tions. In the J -band, we measure a linear flux increase from80 counts on 1990 February 1 to 100 counts on 1993 April 8(0.24 mag). The standard deviation is 0 .
23 mag, with similarvariations in the other bands, consistent with what is observedfor some other LBVs. For example, from 1985 to 1992, AGCar showed a steady small flux increase with variations of 0.1mag (e.g. van Genderen et al. 1997). DISTANCE
We measured an LSR velocity, V LSR , of − ± − using the Mg II lines in the 2013 and 2016 SINFONI datafor which we were able to measure the locations of telluricOH lines in order to set the wavelength scale. This V LSR wasconfirmed, within the uncertainties, by cross-correlating ourspectroscopic model (see Sect.6) with the 2016 spectra, de-noting that the shapes of the Mg II lines are barely a ff ectedby the stellar wind. Note that we validated our wavelengthcalibration technique by applying it to SINFONI data sets forGG Car and MCW 137, both early type supergiants with sig-nificant winds and near-infrared spectral morphology similarto that of IRAS 16115 − V LSR and inferred distances consistent with values in the liter-ature. Using the A5 model of Reid et al. (2014), we estimatea distance of 3.68 ± A K s determined in Section 4 is consistentwith the kinematic distance Messineo et al. (2020). Ra-dio wavelength absorption has been observed in the 1612,1665 and 1667 GHz hyperfine structure lines of the OHmolecule toward the position of of IRAS 16115 − −
100 to −
35 km s − .Urquhart et al. (2007) published a spectrum of the CO J = − ff ort. This line shows various emissionfeatures with LSR velocities between −
109 and −
45 km s − ,similar to the velocities covered by the OH lines. Giventhe ubiquity of OH and CO in the interstellar medium andthe relatively large beam widths used in the radio studies,it is impossible to establish a direct relation of the molec-ular gas with IRAS 16115 − F ig . 1.— The upper panel compares the 2013 (dashed red) and 2016 (solid black) spectra of IRAS 16115 − + of the OH absorption, up to − − − ̟ = ± − , giving a distance range of1.1 to 2 kpc. We give this distance little weight, as comparisonof distances inferred from GAIA data with spectrophotomet- ric distances for OB stars implies deviations up to 50% fordistances > WARM DUST
Two composite images of the region from theGLIMPSE and MIPSGAL surveys are shown in Figure2. IRAS 16115 − µ m source (106 Jy)and is listed in the catalog of Mizuno et al. (2010) as anextended source with a diameter of 47 ′′ , corresponding tophysical size of 0.8 pc for a distance of 3.6 kpc, sizes thatare typical for LBVs (Nota et al. 1995). The object is belowthe detection threshold of van de Steene & Pottasch (1993)at radio wavelengths, suggesting that the star is not ionizingthe nearby dust. This precludes the typical straightforwarddetermination of the nebular mass since any determination Figer et al. TABLE 1P hotometric measurements of
IRAS 16115 − Survey band λ Flux mag Name Reference[ µ m] [Jy] [mag]GSC2.2 R 0.70 0.00 17.15 S230213364722 Lasker et al. (2008)USNOB1 R2 0.70 0.00 17.79 0391-0542333 Monet et al. (2003)USNOB1 Imag 0.90 0.01 13.47 0391-0542333 Monet et al. (2003)DENIS I 0.79 0.01 13.05 J161517.9-505219 Epchtein et al. (1994)2MASS J 1.23 1.25 7.76 16151795-5052197 Cutri et al. (2003)DENIS J 1.22 1.38 7.67 J161517.9-505219 Epchtein et al. (1994)2MASS H 1.63 3.58 6.13 16151795-5052197 Cutri et al. (2003)2MASS K 2.15 5.80 5.14 16151795-5052197 Cutri et al. (2003)GLIMPSE 4.5 4.50 5.38 3.81 G332.2843-00.0008 Churchwell et al. (2009)GLIMPSE 5.8 5.80 3.83 3.69 G332.2843-00.0008 Churchwell et al. (2009)GLIMPSE 8.0 8.00 3.02 3.32 G332.2843-00.0008 Churchwell et al. (2009)WISE W3 11.56 3.31 2.36 J161518.17-505220.4 Wright et al. (2010)Mipsgal F24 23.70 106.00 -2.85 MGE332.2843-00.0002 Gutermuth & Heyer (2015)IRAS F12 12.00 10.40 1.09 IRAS16115 − − − − from the properties of the dusty component would requirea somewhat arbitrary dust:gas ratio to be adopted. We alsonote the apparent asymmetric nature of the nebula at 24 µ m(Figure 2, right). INTERSTELLAR EXTINCTION
IRAS 16115 − L ⊙ ), but this was due to an incorrect es-timate of interstellar extinction.Using the spectral energy distribution (SED) from the spec-troscopic model, we estimate that the intrinsic near-infraredcolors are close to zero ( H − K = J − H = A K s = LUMINOSITY AND MASS
We modeled the SED and K-band spectrum with CMF-GEN (Hillier & Miller 1998) in a process similar to that de-scribed in Najarro et al. (2009), finding log ( L / L ⊙ ) = . T e ff = τ Ross = /
3, R = R ⊙ ), V in f = − , and˙ M = − ) M ⊙ yr − with a moderate clumping (f cl ∼ . / H degeneracy and obtainHe / H = / N compared to the 2016 data. However, we can qualita-tively say that the 2013 spectrum corresponds to a slightlyhigher temperature phase reflected by the He I components,the increased strength of the Fe II lines, and the weakenedNa I lines.Figure 4 plots the location of the object on the HR diagram,along with data points for LBVs and evolutionary tracks for rotating massive stars (Ekstr¨om et al. 2012). From the loca-tion of IRAS 16115 − ≈ M ⊙ , assuming that the ob-ject is a single star. While it is possible that it could be amultiple star system, we note that there are no indications ofmultiplicity, or binary star evolution, in the spectra. CONCLUSIONS
We revise the classification of IRAS 16115 − K -bandspectra and photometry. Photometric variations of ≈
20% aredetected in DIRBE data, while mid-infrared imaging con-firms the presence of a circumstellar nebula. The spectro-scopic variability exhibited is replicated almost exactly by thebona fide LBV qF362. We estimate a distance of 3.68 kpc, log ( L / L ⊙ ) = .
75, and temperature in the range of 10.5 to13 kK. From the luminosity and temperature, along with amodel, we infer an initial mass of ≈ M ⊙ . All of the ob-served and inferred properties are similar to those of well-established LBVs. We consider IRAS 16115 − ff orts to produce the data fromIRAS, DIRBE, GSC, USNO, 2MASS, DENIS, GLIMPSE,MIPSGAL, AKARI, and WISE surveys, and the Sci-ence Archive of the European Southern Observatory. Wethank John Hillier for providing the CMFGEN code andJames Urquhart for information on the radio emissionfrom IRAS 16115 − / MCIU / AEI / FEDER) and from the SpanishState Research Agency (AEI) through the Unidad de Exce-lencia Mara de Maeztu-Centro de Astrobiologa (CSIC-INTA)project No. MDM-2017-0737.
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