Two Nested Shells around the Blue Supergiant ALS 19653
aa r X i v : . [ a s t r o - ph . S R ] S e p Draft version October 2, 2020
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Two nested shells around the blue supergiant ALS 19653
J.M. Drudis and V.V. Gvaramadze
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Curiosity2 Observatory, New Mexico Skies, Mayhill, NM 88339, USA Sternberg Astronomical Institute, Lomonosov Moscow State University, Universitetsky pr., 13, Moscow 119234, Russia Space Research Institute, Russian Academy of Sciences, Profsoyuznaya 84/32, 117997 Moscow, Russia
Keywords: circumstellar matter — stars: individual (ALS 19653) — stars: massive — supergiantsABSTRACTWe present the results of deep narrowband imaging of two nested shells around the blue supergiant ALS 19653,which confirm that the outer shell is physically associated with the star.INTRODUCTIONMassive stars experience episodes of intense mass loss during their lifetime, leading to the formation of parsec-scalecircumstellar nebulae of various forms. Searching for such nebulae serves as an effective way to detect rare types ofmassive stars, such as the luminous blue variables, blue supergiants and Wolf-Rayet stars (e.g. Gvaramadze et al.2010, 2012). One of the massive stars revealed in this way is the central star, ALS 19653, of a dumbbell-like infrarednebula discovered (Gvaramadze et al. 2019, hereafter Paper I) in the archival data of the
Wide-field Infrared SurveyExplorer ( WISE ; Wright et al. 2010). Follow-up inspection of H α +[N ii ] images from the Isaac Newton Telescope(INT) Photometric H α Survey of the Northern Galactic Plane (IPHAS; Drew et al. 2005) lead to the detection ofan almost circular shell (of angular radius of ≈ . ≈ . ≈ . ≈ . ≈ −
30 and 100 km s − , respectively (Paper I). Itwas also found that the spectrum of the inner shell shows only emission lines of H α and [N ii ] λλ iii ] emission. The main purpose of the study presented inthis note is the deep narrowband imaging of the environs of ALS 9653, aimed to get a better idea of the morphologyof the outer shell and to confirm its connection to the star.NARROWBAND IMAGINGTo fulfill our purpose, a deep color image was planned to be taken with the 61 cm telescope (f/6.5; camera FLIPL16803 16MPx, 9 µ ) at the Curiosity2 Observatory using narrowband (FWHM 15 ˚A) H α , [N ii ] and [O iii ] filterscentered, respectively, at 6563, 6584 and 5007 ˚A. The exposure times for individual images taken with these filterswere set at 40 min each. After taking 37 H α and 26 [O iii ] images (total exposure times of ≈
25 and 17 hours,respectively), it became clear that, on one side, our H α image (Figure 1(b)) did not lack any detail that was alreadypresent in the IPHAS image. This was a strong indication that there was no specific [N ii ] emission in this nebula, andwe then dropped the capture of any frames with the [N ii ] filter. The second conclusion was that the [O iii ] imagesdid not show any traces of neither the inner nebula (which is consistent with the lack of the [O iii ] λ α (red) image and a set of short exposure frames through the broadband R , G and B filters in order to depict the stars inthe image with their true natural colors. The processing of this image was very simple in order to preserve the originalstructures. The only treatment was a dual deconvolution applied on different parts of the image. A maximum entropy Corresponding author: J.M. [email protected] deconvolution was applied to the outer shell, in order to enhance its presence, and a positive constraint deconvolutionwas applied to the central nebula in order to obtain greater detail of its structure. Figure 1(b) shows the outer shellwithout applying the maximum entropy deconvolution, in order to show its real brightness, compared to the centralnebula.Figure 1(a) shows that the outer shell represents an almost circular nebula whose emission fills nearly the entirespace between its outer edge and the central nebula. One can also see that the maximum brightness of the shell fallson its north-east edge and that ALS 19653 is somewhat offset towards this edge. This could be caused either by anorth-east motion of the star relative to the ambient medium or by a density gradient in the same direction.SPACE VELOCITY OF ALS 19653To check whether the space motion of ALS 19653 is the cause of the increased brightness of the north-east edgeof the outer shell, we calculated the peculiar transverse velocity, v tr , of this star using its Gaia
DR2 proper motion( µ α cos δ = − . ± .
113 mas yr − , µ δ = − . ± .
112 mas yr − ) and parallax (0 . ± .
063 mas) measure-ments (Gaia Collaboration et al. 2018). To do this, we adopted the solar Galactocentric distance of R = 8 . = 240 km s − (Reid et al. 2009), and the solar peculiar motion of( U ⊙ , V ⊙ , W ⊙ ) = (11 . , . , .
3) km s − (Sch¨onrich et al. 2010). We derived the peculiar velocity components alongthe Galactic longitude and latitude of v l = 8 . ± . − and v b = 1 . ± . − , respectively, which correspond to v tr = 8 . ± . − . Using the systemic radial velocity of ALS 19653 of − . ± . − (Paper I), we calculated alsothe peculiar radial velocity v r = − . ± . − and the total space velocity of this star of v tot = 19 . ± .
36 km s − .The quoted uncertainties in the velocity estimates take into account only the uncertainties in the proper motion andsystemic velocity measurements. Although the derived value of the space velocity did not allow us to treat ALS 19653as a classical runaway star, its orientation in the sky shows that this star is moving in the “correct” (north-east)direction, meaning that the enhanced brightness of the north-east edge of the outer shell could indeed be caused bythis motion. DISCUSSIONDetection of two nested shells around ALS 19653 indicates that this star at least twice went through episodes ofenhanced mass lose in the recent ( ∼ yr) past. Similar two-component nebulae were also found around both massiveand low-mass stars, e.g., the O6f?p star HD 148937 (Leitherer & Chavarria-K 1987) and the M1 II star HD 65750(Drudis 2018). The common characteristic of these objects is that their inner nebulae possess axial symmetry.Although the mechanisms that lead to the formation of such nebulae are not fully understood, it is believed that theyare somehow related to the present or past binarity of their central stars. In particular, the fast rotation and strongmagnetic field of HD 148937 and the nitrogen-rich composition of its inner nebula make this star a strong mergercandidate (Langer 2012). Similarly, it was suggested that ALS 19653 is a merger product as well (Paper I). Thepossible binarity of these two stars (Wade et al. 2019; Gvaramadze et al. 2019) does not contradict these suggestionsbecause both stars could originally be members of triple or higher multiple systems (cf. Langer 2012).Finally, we note that a careful inspection of the WISE µ m image revealed that the central nebula is surroundedby a diffuse halo of the same angular extent as the outer optical shell (Figures 1(c) and 1(d)), which provides furtherevidence of the connection of this shell with ALS 19653.V.V.G. acknowledges support from the Russian Foundation for Basic Research grant 19-02-00779. Figure 1.
Figure 1(a) (upper left): final color image. Figure 1(b) (upper right): inverted H α image. Figures 1(c) and 1(d)(bottom panels): H α (left) and WISE µ m (right) images. The orientation and the scale of all images are the same. REFERENCES
Drew J., Greimel R., Irwin M. J. et al
MNRAS
RNAAS et al
A&A
616 A1Gvaramadze V. V., Kniazev A. Y. and Fabrika S. 2010
MNRAS
405 1047Gvaramadze V. V., Kniazev A. Y., Miroshnichenko A. S. etal
MNRAS
421 3325Gvaramadze V. V., Kniazev A. Y., Castro N. and Grebel E.K. 2019 AJ
157 53 Langer N. 2012
ARA&A
50 107Leitherer C. and Chavarria-K C. 1987
A&A
175 208Reid M. J., Menten K. M., Zheng X. W., Brunthaler A.and Xu Y. 2009
ApJ
705 1548Sch¨onrich R., Binney J. and Dehnen W. 2010
MNRAS et al
MNRAS