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We present time-series imaging polarimetry observations of a very nearby superluminous supernova 2018hti at z = 0.063, to probe its explosion mechanism and shed light on the energy source powering superluminous supernovae (SLSNe). We obtain linear polarimetry using the Alhambra Faint Object Spectrograph and Camera on board the 2.5 m Nordic Optical Telescope. Our observations show polarization at the ~1.9% level before and after the supernova reached light-curve maximum. However, this level of polarization is consistent with the line-of-sight interstellar polarization; hence, the polarization from the supernovae itself was not detected at a level of >3σ. This suggests that the explosion of SN 2018hti is almost spherical, similar to previous polarimetry studies of other SLSNe.

We obtained time-series spectroscopy of the G0 subgiant eta Boo in an attempt to confirm the solar-like oscillations reported by Kjeldsen et al. (1995). We recorded 1843 spectra over six consecutive nights with the Nordic Optical Telescope, which we used to measure equivalent widths of strong temperature-sensitive lines. We also measured velocities from 1989 spectra obtained through an iodine reference cell at Lick Observatory over 56 nights that were badly affected by weather. Our analysis also included velocity measurements published by Brown et al. (1997) and the original equivalent-width measurements by Kjeldsen et al. (1995). All four data sets show power excesses consistent with oscillations, although with a range of amplitudes that presumably reflects the stochastic nature of the excitation. The highest peaks show regularity with a large separation of Delta nu = 40.4 microHz and we identify 21 oscillation frequencies from the combined data., Comment: 18 pages, LaTeX with figures. Accepted for publication by AJ

We thank Robert Wilson (University of Florida) for providing a custom version of his code to compute images of spotted stellar surfaces and for his help with using it. This work is based upon observations obtained with the Georgia State University (GSU) Center for High Angular Resolution Astronomy (CHARA) array at Mount Wilson Observatory. The CHARA array is supported by the National Science Foundation under grant numbers AST-1211929 and AST-1411654. Institutional support has been provided by the GSU College of Arts and Sciences and the GSU Office of the Vice President for Research and Economic Development. The MIRC instrument at the CHARA array was funded by the University of Michigan. F.B., R.R., and J.D.M. acknowledge support from NSF-AST 1210972 and 1108963. G.T. acknowledges partial support from NSF grant AST-1509375. S.K. acknowledges support from an STFC Rutherford Fellowship (ST/J004030/1) and ERC Starting Grant (grant agreement no. 639889). This work is also based on observations made with the Nordic Optical Telescope (NOT), operated by the Nordic Optical Telescope Scientific Association at the Observatorio del Roque de los Muchachos, La Palma, Spain, of the Instituto de Astrofisica de Canarias. This research has made use of the SIMBAD database, operated at the CDS, Strasbourg, France. This research has made use of the Jean-Marie Mariotti Center SearchCal service13 codeveloped by FIZEAU and LAOG/IPAG and of the CDS astronomical databases SIMBAD and VIZIER.14 This research has made use of the Washington Double Star Catalog, maintained at the U.S. Naval Observatory. We thank Nicholas Elias II for discussions. We thank Dimitri Pourbaix for maintaining and providing access to the SB9 database of RV measurements of spectroscopic binaries.

We present imaging polarimetry of the superluminous supernova SN 2015bn, obtained over nine epochs between $-$20 and $+$46 days with the Nordic Optical Telescope. This was a nearby, slowly-evolving Type I superluminous supernova that has been studied extensively and for which two epochs of spectropolarimetry are also available. Based on field stars, we determine the interstellar polarisation in the Galaxy to be negligible. The polarisation of SN 2015bn shows a statistically significant increase during the last epochs, confirming previous findings. Our well-sampled imaging polarimetry series allows us to determine that this increase (from $\sim 0.54\%$ to $\gtrsim 1.10\%$) coincides in time with rapid changes that took place in the optical spectrum. We conclude that the supernova underwent a `phase transition' at around $+$20 days, when the photospheric emission shifted from an outer layer, dominated by natal C and O, to a more aspherical inner core, dominated by freshly nucleosynthesized material. This two-layered model might account for the characteristic appearance and properties of Type I superluminous supernovae., Comment: ApJ Letters, accepted

Optical region spectra of the chemically peculiar HgMn type star HR 7775 have been analyzed for the determination of elemental abundances and spectral line identification as a complementary study to the analysis of Hubble Space Telescope Goddard High Resolution Spectrograph spectra. The spectra were obtained with the SOFIN echelle spectrograph on the Nordic Optical Telescope at nominal resolving powers of R = 25,000, 80,000, and 170,000 and signal-to-noise ratios typically in excess of 100 : 1. The overall nature of the elemental abundance distribution is similar to other cool, HgMn stars; somewhat enhanced abundances for the iron-group elements, with progressively stronger enhancements for heavier elements. Of particular note are the presence of lines from the rare-earth spectra Nd III and Pr III, and the heavy elements Pt, Au, Hg, Tl, and Bi. The isotopic mixture for several lines of mercury and platinum has been investigated. For mercury a possible difference is found between the mixtures as derived from lines of Hg I and Hg II. Three lines of Pt II display the same mixture, which is different from that found from lower excitation lines at ultraviolet wavelengths. We present new laboratory Fourier Transform spectrometer measurements for isotopic and hyperfine structure components of Hg I λ4046 and 4358 and Hg II λ3984 A.

Optical Transients from gamma-ray burst sources, in addition to offering a distance determination, convey important information on the physics of the emission mechanism, and perhaps also about the underlying energy source. As the gamma-ray phenomenon is extremely diverse, with time scales spanning several orders of magnitude, some diversity in optical counterpart signatures appears plausible. We have studied the Optical Transient, which accompanied the gamma-ray burst of May 8, 1997 (GRB 970508). Observations conducted at the 2.5-m Nordic Optical Telescope (NOT) and the 2.2-m telescope at the German-Spanish Calar Alto observatory (CAHA) cover the time interval starting 3 hours 5 minutes to 96 days after the high energy event. This brackets all other published observations, including radio. When analyzed in conjunction with optical data from other observatories, evidence emerges for a composite light curve. The first interval, from 3 to 8 hours after the event was characterized by a constant, or slowly declining brightness. At a later moment the brightness started increasing rapidly, and reached a maximum approximately 40 hours after the GRB. From that moment the GRB brightness decayed approximately as a power-law of index -1.21. The last observation, after 96 days, m_R = 24.28+-0.10, is brighter than the extrapolated power-law, and hints that a constant component, m_R = 25.50+-0.40 is present. The OT is unresolved (FWHM 0.83") at the faintest magnitude level. The brightness of the optical transient, its duration and the general shape of the light curve sets this source apart from the single other optical transient known, that of the February 28, 1997 event., Comment: 13 pages, LaTeX (aaspp4.sty) 2 figures. To appear in ApJ 496 (March 20, 1998)

We report the discovery of Kepler-15b (KOI-128), a new transiting exoplanet detected by NASA's Kepler mission. The transit signal with a period of 4.94 days was detected in the quarter 1 (Q1) Kepler photometry. For the first time, we have used the High Resolution Spectrograph (HRS) at the Hobby-Eberly Telescope (HET) to determine the mass of a Kepler planet via precise radial velocity (RV) measurements. The 24 HET/HRS RVs and 6 additional measurements from the Fibre-fed Échelle Spectrograph spectrograph at the Nordic Optical Telescope reveal a Doppler signal with the same period and phase as the transit ephemeris. We used one HET/HRS spectrum of Kepler-15 taken without the iodine cell to determine accurate stellar parameters. The host star is a metal-rich ([Fe/H] = 0.36 ± 0.07) G-type main-sequence star with T_eff = 5515 ± 124 K. The semi-amplitude K of the RV orbit is 78.7^(+8.5)_(–9.5) m s^(–1), which yields a planet mass of 0.66 ± 0.1 M_Jup. The planet has a radius of 0.96 ± 0.06 R_Jup and a mean bulk density of 0.9 ± 0.2 g cm^(–3). The radius of Kepler-15b is smaller than the majority of transiting planets with similar mass and irradiation level. This suggests that the planet is more enriched in heavy elements than most other transiting giant planets. For Kepler-15b we estimate a heavy element mass of 30-40 M_⊕.