The eruptive young stellar object FU Ori is the eponym of its variable class. FU Ori stars are known to undergo outbursts with amplitudes of gt4 mag in the V band and durations of several decades. Interaction with a binary companion is one proposed outburst trigger so understanding both components of the FU Ori system is crucial. A recent Hubble Space TelescopeSpace Telescope Imaging Spectrograph observation of the FU Ori system clearly resolves its North and South components. We report here on the spectrum of FU Ori South. We detect nearultraviolet continuum emission but no farultraviolet continuum although several bright emission lines consistent with those seen in T Tauri stars are present. The presence of the C II 2325 multiplet and many HSUB2SUB lines indicate active accretion. We estimate the extinction to the source and find that the UV spectrum favors A SUB V SUB lt 4 contrary to past estimates based on the nearinfrared spectrum.
['FU Orionis stars', 'Classical T Tauri stars', 'Stellar accretion', 'Ultraviolet spectroscopy', '553', '252', '1578', '2284', 'Astrophysics - Solar and Stellar Astrophysics']
The Farultraviolet Spectrum of FU Ori South
{'The FUV Spectrum of FU Ori South': "ADOLFO S. CARVALHO, 1 GREGORY J. HERCZEG, 2, 3 KEVIN FRANCE, 4 AND LYNNE A. HILLENBRAND 1  \n1 Department of Astronomy; California Institute of Technology; Pasadena, CA 91125, USA  \n2 Kavli Institute for Astronomy and Astrophysics, Peking University, Beijing 100871, People's Republic of China 3 Department of Astronomy, Peking University, Beijing 100871, People's Republic of China  \n4 Laboratory for Atmospheric and Space Physics, University of Colorado Boulder, Boulder, CO 80303, USA", 'ABSTRACT': 'The eruptive YSO FU Ori is the eponym of its variable class. FU Ori stars are known to undergo outbursts with amplitudes of > 4 magnitudes in the V band and durations of several decades. Interaction with a binary companion is one proposed outburst trigger, so understanding both components of the FU Ori system is crucial. A recent HST/STIS observation of the FU Ori system clearly resolves its North and South components. We report here on the spectrum of FU Ori South. We detect NUV continuum emission but no FUV continuum, although several bright emission lines consistent with those seen in T Tauri stars are present. The presence of the C II] 2325 multiplet and many H 2 lines indicate active accretion. We estimate the extinction to the source and find that the UV spectrum favors A V < 4 , contrary to past estimates based on the NIR spectrum.', '1. INTRODUCTION': "A popular proposed trigger of FU Ori outbursts is the interaction with a binary companion during the periastron passage of the pair (Bonnell & Bastien 1992). The detection of a companion 0 '' .5 to the south of FU Ori (Wang et al. 2004; Reipurth & Aspin 2004) lent credence to this theory, with Pueyo et al. (2012) confirming its physical association through astrometry. Adaptive-optics-assisted integral field spectroscopy with Gemini North/NIFS (Beck & Aspin 2012) revealed that the near infrared (NIR) spectrum of FU Ori S has atomic absorption features like those in low mass young stellar objects (YSOs) with a K5 spectral type.  \nThe NIR continuum of FU Ori S has a perplexing shape, with the J band indicating significant line-of-sight extinction of A V ≈ 6 mag while the H -K band continuum matches A V = 0 . 5 mag. The Pa β and Br γ emission gives an accretion rate of ˙ M = 3 × 10 -8 M ⊙ yr -1 (Beck & Aspin 2012), typical of low-mass Class II YSOs (Manara et al. 2023). The CO ∆ ν = 2 bandheads in K band are all in emission, as has been seen in some rapidly accreting Class I/II YSOs (e.g., Doppmann et al. 2008).  \nPueyo et al. (2012) combined a low-resolution Palomar/P1640 spectrum of FU Ori S with MIR photometry and SED fitting to find a stellar temperature range of T eff = 4000 -5000 K, consistent with Beck & Aspin (2012) but requiring higher extinction values ( A V = 8 -12 mag). The conflicting A V estimates may be due to the exotic sightline to the source, which includes significant contribution from the disk and accretion streamers around FU Ori N (Takami et al. 2018; P'erez et al. 2020; Hales et al. 2024).  \nWe present in this Note the first far- and near- ultraviolet (FUV/NUV) spectrum of FU Ori S, which shows emission lines that are consistent with those seen in accreting low mass young stars. We compare the spectrum with the wellknown source BP Tau, which is a typical classical T Tauri Star (CTTS) with a similar spectral type and mass accretion rate (Manara et al. 2023) to FU Ori S.", '2. DATA AND CALIBRATION': "We obtained spectra of the FU Ori system using the Space Telescope Imaging Spectrograph (STIS) as part of HST Guest Observer program 17176 (PI: L. Hillenbrand). The observation was taken using the 52 '' X2 '' slit in the G140L (FUV-MAMA), G230L (NUV-MAMA), and G430L (CCD) grating settings. The spectra are available on the Mikulski Archive for Space Telescopes (MAST) and can be accessed via doi: 10.17909/3p42-jw31. We only see the South component in the two bluest spectra.  \nIn the FUV and NUV spectra, the STIS position angle (PA) is -131 · and the 2 '' slit captures the entire system. The separation of ∼ 0 . 2 '' between the North and South traces in the x2d spectrum is consistent with the 0 . 48 '' and 163 · binary separation and PA (Pueyo et al. 2012) projected to the slit PA. The traces are clearly separated and can be extracted individually. The automatic extraction in MAST included only the primary, so we performed aperture extraction of the secondary. We flux-calibrated using a sensitivity function obtained from previous observations. The final spectrum of FU Ori S covers 1100-3200 ˚ A with R ≡ λ/ ∆ λ = 600 . Between 1700 ˚ A and 2200 ˚ A the observations are consistent  \nwith noise. The spatial offset of FU Ori S produces a ≈ 5 ˚ A blueshift in the wavelength solution.  \nThe spectrum of BP Tau was obtained in 2002 in the HST GOprogram 9081 (PI: N. Calvet; Kravtsova & Lamzin 2003) and was accessed through MAST.", '3. THE UV SPECTRUM OF FU ORI S': 'The spectrum of FU Ori S is shown in Figure 1, alongside that of BP Tau. Although the FUV spectrum is not sensitive enough to detect continuum, there are several emission lines that correspond well to those in BP Tau. In particular, Ly α , C II 1334.5/1335.7, Si IV 1393.8/1402.8, C IV 1548.2/1550.8, and He II 1640 are all clearly detected.  \nThe NUV spectrum is higher signal-to-noise and shows both continuum and line emission. The NUV C II] 2325 multiplet, Si II] 2350 line, Al III] 2670 line, and the Mg II 2796/2803 doublet are all prominent and there is a broad emission feature blue-ward of the Mg II doublet centered at 2775 ˚ A with a Gaussian FWHM = 54 ˚ A.  \nThe presence of the C II] 2325 feature is an indicator of accretion in FU Ori S (Ingleby et al. 2013), so we sought to use it to estimate the L acc in the system. First, we used the fit continuum function in the specutils Python package to fit a continuum to the spectrum using two regions: 2100 ˚ A to 2200 ˚ A and 2400 ˚ A to 2450 ˚ A. We then fit a double Gaussian profile to the continuum-subtracted line.  \nWe adopt the 404 pc distance to the source (Kounkel et al. 2018; Roychowdhury & Hillenbrand 2024) and use the Whittet et al. (2004) extinction law for our extinction corrections. We report the calculated line luminosities assuming both A V = 0 mag and A V = 2 . 5 mag, the value used by Beck & Aspin (2012). We measure a line flux of 1 . 7 × 10 -14 erg s -1 cm -2 ( 5 . 2 × 10 -12 erg s -1 cm -2 ) in the multiplet and a luminosity of 8 . 4 × 10 -5 L ⊙ ( 0 . 025 L ⊙ ) assuming A V = 0 mag (2.5 mag). With the Ingleby et al. (2013) calibration, log 10 ( L acc ) = 1 . 1 log 10 ( L CII] ) + 2 . 7 , we estimate L acc = 0 . 02 L ⊙ ( L acc = 9 L ⊙ ). The luminosity for A V = 2 . 5 mag is almost 50 × greater than the L acc = 0 . 2 L ⊙ reported by Beck & Aspin (2012).  \nFollowing the same procedure for the C IV 1548/1550 doublet, we compute an integrated line flux of 3 . 2 × 10 -15 erg s -1 cm -2 ( 1 . 1 × 10 -12 erg s -1 cm -2 ), which is L CIV = 1 . 6 × 10 -5 L ⊙ ( 0 . 005 L ⊙ ), assuming A V = 0 mag (2.5 mag). Using the calibration from Yang et al. (2012), we es-  \nan accretion luminosity of 0 . 005 L ⊙ ( 3 . 90 L ⊙ ). For A V = 2 . 5 , the L acc is lower than that estimated from the C II NUV multiplet but still more than an order of magnitude greater the Beck & Aspin (2012) value.  \nWe detect strong emission from Mg II 2796/2803 doublet, but it is a less reliable tracer of accretion due to heavy outflow absorption (Ardila et al. 2002) and its presence in nonaccreting T Tauri star spectra (Ingleby et al. 2013).  \nThe uncertainty in the A V to the source translates to uncertainties in L acc spanning several orders of magnitude. One potential means of estimating the A V to FU Ori S from the UV spectrum, independently of existing values derived from the NIR SED, is to use the spectrum of BP Tau as a template. We vary the extinction correction for FU Ori S, normalizing both spectra to 1 at 3000 ˚ A at each step, to find which extinction correction gives the best match to the UV slope of BP Tau. The best-matching relative extinction correction is ∆ A V = 2 . 0 mag. Given the extinction of A V = 0 . 5 of BP Tau (Herczeg & Hillenbrand 2014), this implies an A V = 2 . 5 mag for FU Ori S, in good agreement with the value adopted by Beck & Aspin (2012). However, the measured accretion luminosities for C II], C IV, and Mg II for this A V are much larger than their estimates with the NIR hydrogen lines.', '4. CONCLUSIONS': 'The UV spectrum of FU Ori S is similar to that of a typical low mass YSO. Although the FUV continuum is undetected and many of the lines are only marginally detected, they are matched to lines seen in another YSO, BP Tau. The NUV is a very good match to the spectrum of BP Tau, with the exception of a broad emission feature at 2775 ˚ A. The bright FUV/NUV emission, along with measurements of the flux from the C II], C IV, and Mg II features indicate that the target is unlikely to be as heavily extincted as previously reported, or that the extinction law departs drastically from that of Whittet et al. (2004). This may be possible since the dominant source of extinction to FU Ori S may be the circumstellar material and disk of FU Ori N.', '5. ACKNOWLEDGEMENTS': 'This research was supported, in part, by grant HST-GO17176.001-A from STScI.', 'REFERENCES': "Ardila, D. R., Basri, G., Walter, F. M., Valenti, J. A., &  \nJohns-Krull, C. M. 2002, ApJ, 567, 1013, doi: 10.1086/338586  \nBeck, T. L., & Aspin, C. 2012, AJ, 143, 55,  \ndoi: 10.1088/0004-6256/143/3/55  \nBonnell, I., & Bastien, P. 1992, ApJL, 401, L31,  \ndoi: 10.1086/186663  \nDoppmann, G. W., Najita, J. R., & Carr, J. S. 2008, ApJ, 685, 298,  \ndoi: 10.1086/590328  \nFigure 1. The UV spectrum of FU Ori S, compared with that of BP Tau. Emission lines commonly seen in the spectra of accreting T Tauri stars are marked in vertical solid light blue lines. The dotted line shows the location of the feature at 2775 ˚ A. Between 1400 ˚ A and 1550 ˚ A the H 2 emission lines are marked by cyan vertical lines, some of which are almost as bright as the C IV doublet at 1548/1550 ˚ A.  \n<!-- image -->  \nHales, A. S., Gupta, A., Ru'ız-Rodr'ıguez, D., et al. 2024, ApJ, 966, 96, doi: 10.3847/1538-4357/ad31a1  \nHerczeg, G. J., & Hillenbrand, L. A. 2014, ApJ, 786, 97,  \ndoi: 10.1088/0004-637X/786/2/97  \nIngleby, L., Calvet, N., Herczeg, G., et al. 2013, ApJ, 767, 112, doi: 10.1088/0004-637X/767/2/112  \nKounkel, M., Covey, K., Su'arez, G., et al. 2018, AJ, 156, 84,  \ndoi: 10.3847/1538-3881/aad1f1  \nKravtsova, A. S., & Lamzin, S. A. 2003, Astronomy Letters, 29,  \n612, doi: 10.1134/1.1607499  \nManara, C. F., Ansdell, M., Rosotti, G. P., et al. 2023, in ASPC,  \nVol. 534, Protostars and Planets VII, ed. S. Inutsuka, Y. Aikawa,  \nT. Muto, K. Tomida, & M. Tamura, 539,  \ndoi: 10.48550/arXiv.2203.09930"}