SOFIA and HST Multi-wavelength study of the Symbiotic Mira HM Sge

HM Sge is a symbiotic system composed of an oxygen-rich AGB star accreting material onto a white dwarf 60 au away. The AGB star has a pulsation period of 527 days [3], an M7 spectral type [4], and is heavily reddened in the near-IR (J − K ∼ 3 mag), all indicative of dust production, high mass loss, and a late stage of AGB evolution. The system is also relaxing from a nova-like outburst that occurred in 1975, which drew much attention and has provided a wealth of archival data. Using new data from the Hubble Space Telescope (COS & WFC3) and SOFIA observatory (EXES & FORCAST) we study the circumstellar dust shell, molecular flows, and the compact nebula surrounding HM Sge’s central engine. We take advantage of the long baseline of observations to study the evolution and properties of the post-outburst environment. We will present preliminary results of our analysis and an update of the status of the system.

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1. SOFIA and HST Multi-wavelength study of the Symbiotic Mira HM Sge Steven Goldman1,2, Ravi Sankrit2, Nicole Karnath1, Ed Montiel1, Matthew Richter3 1 SOFIA-USRA, NASA Ames Research Center, MS 232-12, Moffett Field, CA 94035, USA 2 Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218 USA 3 Department of Physics, University of California Davis, Davis, CA, USA References: [1] Dokuchaeva, O. D. 1976, Information Bulletin on Variable Stars, 1189, 1 [2] Eyres et al. 2001, ApJ, 551, 512 [3] Murset, U., & Nussbaumer, H. 1994, A&A, 282,586 [4] Mürset, U., & Schmid, H. M. 1999, A&AS, 137, 473 [5] Schild et al. 2001, A&A, 378, 146 Figure 3: New FORCAST photometry and grism spectroscopy of HM Sge, along with archival mid-IR spectra from IRAS and ISO [5]. FORCAST: Photometry and spectroscopy in the mid-IR allow us to characterize the silicate dust. By comparing our new FORCAST observations with archival IR data (Fig. 3), we can see how the dust content and properties have evolved as the system has relaxed after its outburst. SOFIA OBSERVATORY (PID: 75_0057) WFC3: We have used high angular resolution HST/WFC3 narrow- band images in Hα, [N ii] and [O iii] to reveal the morphology of HM Sge’s ionized gas and jet. We have additionally used a longer exposure in the F657N filter to reveal the morphology of the extended nebular emission (Fig. 1) . We will examine changes that have occurred since the earlier epochs of observation at sub- arcsecond scales to estimate the velocities of the jet outflow. HUBBLE SPACE TELESCOPE (PID: 16492) Figure 2: Our new COS FUV spectrum of HM Sge along with an archival spectrum from IUE [4]. Figure 1: New and archival[2] HST NUV images of HM Sge. HM Sge is a symbiotic system composed of an oxygen-rich AGB star accreting material onto a white dwarf (WD) 60 au away. The AGB star has a pulsation period of 527 days [3], an M7 spectral type[4], and is heavily reddened in the near-IR (J − K ∼ 3 mag), all indicative of dust production, high mass loss, and a late stage of AGB evolution. The system is also relaxing from a nova-like outburst that occurred in 1975[1], which drew much attention and has provided a wealth of archival data. Using new data from the Hubble Space Telescope (COS & WFC3) and SOFIA observatory (EXES & FORCAST) we study the circumstellar dust shell, molecular flows, and the compact nebula surrounding HM Sge’s central engine. We take advantage of the long baseline of observations to study the evolution and properties of the post-outburst environment. Future Work: A publication outlining the reduced datasets and current status of the system is forthcoming. The data are all public and we encourage its use. 1999 2021 WFPC2/HST NII (F658N) WFC3/HST 1ʺ COS: We have used COS to observe HM Sge in the FUV and NUV covering 915-1950Å and 2800-3200Å (Fig. 2). A rich emission line spectrum is detected, sampling a wide range of ionization states in the gas, from Fe+ and C++ to Ne IV+ and O V+. The presence of highly ionized gas provides evidence of energy input from shocks in addition to photoionization by the hot WD. By comparing the line strengths measured in HST data with those from IUE data obtained almost three decades ago, we will track the long-term evolution of the ionized nebula around the central system. EXES (est. completion March, 2022): High spectral resolution observations with EXES of the H2O molecular lines are used to probe the kinematics of the wind and the accretion flow. Preliminary results show ro-vibrational H2O in emission (Fig 4). Figure 4: A subset of the binned EXES spectrum. Shown in color are a telluric ATRAN model (red) and HITRAN H2O model at 500 K (blue); the HITRAN model is corrected for the systemic velocity of 14.346 km/s. Steve Goldman Observatory Scientist steven.r.goldman@nasa.gov s-goldman.github.io