We present optical light curves, redshifts, and classifications for 365 spectroscopically confirmed Type Ia supernovae (SNe Ia) discovered by the Pan-STARRS1 (PS1) Medium Deep Survey. We detail improvements to the PS1 SN photometry, astrometry and calibration that reduce the systematic uncertainties in the PS1 SN Ia distances. We combine the subset of 279 PS1 SN Ia (0.03 < z < 0.68) with useful distance estimates of SN Ia from SDSS, SNLS, various low-z and HST samples to form the largest combined sample of SN Ia consisting of a total of 1048 SN Ia ranging from 0.01 < z < 2.3, which we call the 'Pantheon Sample'. When combining Planck 2015 CMB measurements with the Pantheon SN sample, we find Ω m = 0.307±0.012 and w = −1.026±0.041 for the wCDM model. When the SN and CMB constraints are combined with constraints from BAO and local H 0 measurements, the analysis yields the most precise measurement of dark energy to date: w 0 = −1.007 ± 0.089 and w a = −0.222 ± 0.407 for the w 0 w a CDM model. Tension with a cosmological constant previously seen in an analysis of PS1 and low-z SNe has diminished after an increase of 2× in the statistics of the PS1 sample, improved calibration and photometry, and stricter light-curve quality cuts. We find the systematic uncertainties in our measurements of dark energy are almost as large as the statistical uncertainties, primarily due to limitations of modeling the low-redshift sample. This must be addressed for future progress in using SN Ia to measure dark energy.

We use high signal-to-noise ratio spectra of 20 H ii regions in the giant spiral galaxy M101 to derive electron temperatures for the H ii regions and robust metal abundances over radii R ¼ 0:19-1.25R 0 (6-41 kpc). We compare the consistency of electron temperatures measured from the [O iii] 4363, [N ii] 5755, [S iii] 6312, and [O ii] 7325 auroral lines. Temperatures from [O iii], [S iii], and [N ii] are correlated with relative offsets that are consistent with expectations from nebular photoionization models. However, the temperatures derived from the [O ii] 7325 line show a large scatter and are nearly uncorrelated with temperatures derived from other ions. We tentatively attribute this result to observational and physical effects, which may introduce large random and systematic errors into abundances derived solely from [O ii] temperatures. Our derived oxygen abundances are well fitted by an exponential distribution over six disk scale lengths, from approximately 1.3 (O/H) in the center to 1/15 (O/H) in the outermost region studied [for solar 12 þ logðO=HÞ ¼ 8:7]. We measure significant radial gradients in N/O and He/H abundance ratios, but relatively constant S/O and Ar/O. Our results are in approximate agreement with previously publishedabundances studies of M101 based on temperature measurements of a few H ii regions. However, our abundances are systematically lower by 0.2-0.5 dex than those derived from the most widely used strong-line '' empirical '' abundance indicators, again consistent with previous studies based on smaller H ii region samples. Independent measurements of the Galactic interstellar oxygen abundance from ultraviolet absorption lines are in good agreement with the T e -based nebular abundances. We suspect that most of the disagreement with the strong-line abundances arises from uncertainties in the nebular models that are used to calibrate the '' empirical '' scale, and that strong-line abundances derived for H ii regions and emission-line galaxies are as much as a factor of 2 higher than the actual oxygen abundances. However, other explanations, such as the effects of temperature fluctuations on the auroral line based abundances, cannot be completely ruled out. These results point to the need for direct abundance determinations of a larger sample of extragalactic H ii regions, especially for objects more metal-rich than solar.

We report extensive observational data for five of the lowest redshift Super-Luminous Type Ic Supernovae (SL-SNe Ic) discovered to date, namely, PTF10hgi, SN2011ke, PTF11rks, SN2011kf, and SN2012il. Photometric imaging of the transients at +50 to +230 days after peak combined with host galaxy subtraction reveals a luminous tail phase for four of these SL-SNe. A high-resolution, optical, and near-infrared spectrum from xshooter provides detection of a broad He i λ10830 emission line in the spectrum (+50 days) of SN2012il, revealing that at least some SL-SNe Ic are not completely helium-free. At first sight, the tail luminosity decline rates that we measure are consistent with the radioactive decay of 56 Co, and would require 1-4 M of 56 Ni to produce the luminosity. These 56 Ni masses cannot be made consistent with the short diffusion times at peak, and indeed are insufficient to power the peak luminosity. We instead favor energy deposition by newborn magnetars as the power source for these objects. A semi-analytical diffusion model with energy input from the spin-down of a magnetar reproduces the extensive light curve data well. The model predictions of ejecta velocities and temperatures which are required are in reasonable agreement with those determined from our observations. We derive magnetar energies of 0.4 E(10 51 erg) 6.9 and ejecta masses of 2.3 M ej (M ) 8.6. The sample of five SL-SNe Ic presented here, combined with SN 2010gx-the best sampled SL-SNe Ic so far-points toward an explosion driven by a magnetar as a viable explanation for all SL-SNe Ic.

In the era of precision cosmology, it is essential to determine the Hubble constant to an accuracy of three per cent or better. At present, its uncertainty is dominated by the uncertainty in the distance to the Large Magellanic Cloud (LMC), which, being our second-closest galaxy, serves as the best anchor point for the cosmic distance scale. Observations of eclipsing binaries offer a unique opportunity to measure stellar parameters and distances precisely and accurately. The eclipsing-binary method was previously applied to the LMC, but the accuracy of the distance results was lessened by the need to model the bright, early-type systems used in those studies. Here we report determinations of the distances to eight long-period, late-type eclipsing systems in the LMC, composed of cool, giant stars. For these systems, we can accurately measure both the linear and the angular sizes of their components and avoid the most important problems related to the hot, early-type systems. The LMC distance that we derive from these systems (49.97 ± 0.19 (statistical) ± 1.11 (systematic) kiloparsecs) is accurate to 2.2 per cent and provides a firm base for a 3-per-cent determination of the Hubble constant, with prospects for improvement to 2 per cent in the future.

In the era of precision cosmology, it is essential to empirically determine the Hubble constant with an accuracy of one per cent or better 1 . At present, the uncertainty on this constant is dominated by the uncertainty in the calibration of the Cepheid period -luminosity relationship 2, 3 (also known as Leavitt Law). The Large Magellanic Cloud has traditionally served as the best galaxy with which to calibrate Cepheid period-luminosity relations, and as a result has become the best anchor point for the cosmic distance scale 4,5 . Eclipsing binary systems composed of late-type stars offer the most precise and accurate way to measure the distance to the Large Magellanic Cloud. Currently the limit of the precision attainable with this technique is about two per cent, and is set by the precision of the existing calibrations of the surface brightness -colour relation 5,6 . Here we report the calibration of the surface brightness-colour relation with a precision of 0.8 per cent. We use this calibration to determine the geometrical distance to the Large Magellanic Cloud that is precise to 1 per cent based on 20 eclipsing binary systems. The final distane is 49.59 ± 0.09 (statistical) ± 0.54 (systematic) kiloparsecs.All data are available upon request from G.P. Extended DataFig.1. Comparison of our relation with the relation of Di Benedetto obtained for giant stars 6 . Top panel, comparison of relations: data points show our results, with the fitted line shown in blue. The blue shaded area represents our obtained r.m.s. scatter of 0.018 mag. The green line is from ref. 6 . Very good agreement is demonstrated. Both S V and (V − K) 0 are in magnitudes. S V physically corresponds to the V band magnitude of a red giant star whose angular diameter is 1 mas. The error bars correspond to 1σ errors. Bottom panel, observed minus calculated values. Extended Data Fig.2. Observed minus calculated surface brightness versus metallicity 6 , [Fe/H]. In a relatively large range of metallicities (about 1 dex) no correlation is found. A formal linear fit gives O − C = 0.0009[Fe/H] -0.002 dex with coefficient of determination R 2 = 0.0001. Fig.3. Example of Monte Carlo simulations for one of our objects, ECL-12669. We computed 10,000 models with the JKTEBOP code 77 from which we obtained statistical uncertainties on the radii R 1 and R 2 , the orbital inclination i, the phase shift φ, the surface brightness ratio j 21 , radial velocity semi-amplitudes K 1 and K 2 , and the systemic velocities γ 1 and γ 2 . For every model we computed the distance modulus converting j 21 into temperature ratio T 2 /T 1 by using Popper's calibration 78 and our original solution with the Wilson-Devinney code 79 . We plot the number of calculated models versus distance modulus (m − M). The dashed line is the best fitted Gaussian and the blue line is the distance determined for this object. The intrinsic (V − K) 0 colours used to estimate the angular diameters of the components were computed using a temperature-colour calibration 28 . Extended DataExtended Data...

The high luminosity and slow decline of their light curves ( Fig PTF12dam is not detected in z P1 images on 1 January 2012, 132 days before the peak.Although their light curves match the declining phases of SN 2007bi and the PISN models quite well, PTF12dam and PS1-11ap rise to maximum light a factor of ~2 faster than these models.The spectra of PTF12dam and PS1-11ap show them to be similar supernovae. After 50 days from the respective light curve peaks, these spectra are almost identical to that of SN 2007bi at the same epoch ( Particularly around and after maximum light, PISN colours are expected to evolve to the red owing to increasing blanketing by iron group elements 7,8 abundant in their ejecta. We see no evidence of line blanketing in our spectra, even down to 2,000 Å (rest frame) in PS1-11ap, which suggests lower iron group abundances and a higher degree of ionization than in PISN models. Such conditions are fulfilled in models of ejecta reheated by magnetars-highly magnetic, rapidly rotating nascent pulsars 13,16,17 . The pressure of the magnetar wind on the inner ejecta can form a dense shell 13,14,17 at near-constant photospheric velocity. ForPTF12dam, the velocities of spectral lines are close to 10,000 km s −1 at all times. Intriguingly, Page 4 of 26 the early spectra of our objects are very similar to those of superluminous supernovae of type I (refs 2, 11, 12) and evolve in the same way, but on longer timescales and with lower line velocities (Fig. 2).Nebular modelling of SN 2007bi spectra has been used to argue 1 for large ejected oxygen and magnesium masses of 8-15M ! and 0.07-0.13M ! , respectively (where M ! is the solar mass). Such masses are actually closer to values in massive core-collapse models 18 than in PISN models, which eject ~40M ! oxygen and ~4M ! magnesium 1,8,9 . In the work reported in ref.1, an additional 37M ! in total of Ne, Si, S, and Ar were added to the model, providing a total ejecta mass consistent with a PISN. However, this was not directly measured 1 , because these elements lack any identified lines. These constraints are important, so we investigated line formation in this phase using our own non-local thermodynamic equilibrium code We suggest here one model that can consistently explain the data. A magnetarpowered supernova can produce a light curve with the observed rise and decline rates as the neutron star spins down and reheats the ejecta 13,14,16,17 . It has been suggested that ~10% of core-collapses may form magnetars 14 . Although their initial-spin distribution is unknown, periods ≳1 ms are physically plausible. This mechanism has already been proposed for SN (Fig. 4), and found a good fit for magnetic field B ≈ 10 14 G and spin period P ≈ 2.6 ms, with an ejecta mass of ~10-16M ! . At peak, the r-band luminosities of PTF12dam and PS1-11ap are ~1.5 times that of SN 2007bi. Scaling our light curve by this factor, our model implies a similar ejected mass for SN 2007bi, with a slower-spinning magnetar (P ≈ 3.3 ms), comparable to previous models 14 . If the mag...

We have obtained new spectrophotometric data for 28 H II regions in the spiral galaxy NGC 300, a member of the nearby Sculptor Group. The detection of several auroral lines, including [O III] λ4363, [S III] λ6312 and [N II] λ5755, has allowed us to measure electron temperatures and direct chemical abundances for the whole sample. We determine for the first time in this galaxy a radial gas-phase oxygen abundance gradient based solely on auroral lines, and obtain the following least-square solution: 12 + log(O/H) = 8.57 (±0.02) − 0.41 (±0.03) R/R 25 , where the galactocentric distance is expressed in terms of the isophotal radius R 25 . The characteristic oxygen abundance, measured at 0.4×R 25 , is 12 + log(O/H) = 8.41. The gradient corresponds to −0.077 ± 0.006 dex kpc −1 , and agrees very well with the galactocentric trend in metallicity obtained for 29 B and A supergiants in the same galaxy, −0.081 ± 0.011 dex kpc −1 . The intercept of the regression for the nebular data virtually coincides with the intercept obtained from the stellar data, which is 8.59 (±0.05). This allows little room for depletion of nebular oxygen onto dust grains, although in this kind of comparison we are somewhat limited by systematic uncertainties, such as those related to the atomic parameters used to derive the chemical compositions.We discuss the implications of our result with regard to strong-line abundance indicators commonly used to estimate the chemical compositions of star-forming galaxies, such as R 23 . By applying a few popular calibrations of these indices based on grids of photoionization models on the NGC 300 H II region fluxes we find metallicities that are higher by 0.3 dex (a factor of two) or more relative to our nebular (T e -based) and stellar ones.We detect Wolf-Rayet stellar emission features in ∼1/3 of our H II region spectra, and find that in one of the nebulae hosting these hot stars the ionizing field has a particularly hard spectrum, as gauged by the 'softness'We suggest that this is related to the presence of an early WN star. By considering a larger sample of extragalactic H II regions we confirm, using direct abundance measurements, previous findings of a metallicity dependence of η, in the sense that softer stellar continua are found at high metallicity.

We present very deep spectrophotometry of 14 bright extragalactic H II regions belonging to spiral, irregular, and blue compact galaxies. The data for 13 objects were taken with the HIRES echelle spectrograph on the Keck I telescope.We have measured C II recombination lines in 10 of the objects and O II recombination lines in 8 of them. We have determined electron temperatures from line ratios of several ions, specially of low ionization potential ones. We have found a rather tight linear empirical relation between T e ([N II]) and T e ([O III]). We have found that O II lines give always larger abundances than [O III] lines. Moreover, the difference of both O ++ abundance determinations -the so-called abundance discrepancy factor-is very similar in all the objects, with a mean value of 0.26 ± 0.09 dex, independently of the properties of the H II region and of the parent galaxy. Using the observed recombination lines, we have determined the O, C, and C/O radial abundance gradients for 3 spiral galaxies: M 33, M 101, and NGC 2403, finding that C abundance gradients are always steeper than those of O, producing negative C/O gradients accross the galactic disks. This result is similar to that found in the Milky Way and has important implications for chemical evolution models and the nucleosynthesis of C.where the auroral lines become very faint, or to measure recombination lines (hereafter RLs) useful for abundance determinations of heavy-element ions (Peimbert 2003;López-Sánchez et al. 2007;Bresolin 2007).The detection of C II and O II lines produced by pure recombination in EHRs was firstly reported by Esteban et al. (2002) from deep spectra taken with the 4.2 m William Herschel Telescope. In principle, these lines have the advantage that their intensity is much less dependent on the value of T e than the collisionally excited lines (hereafter CELs), which are the lines commonly used for abundance determinations in nebulae. The brightest C II RL is C II λ4267, with typical fluxes of the order of 10 −3 × I(Hβ). This line permits to derive the C ++ abundance, which is the dominant ionization stage of C for the typical conditions of EHRs. There are only a few C abundance determinations available for EHRs, most of them derived from UV CELs that can only be observed from space (Garnett et al. 1995(Garnett et al. , 1999Kobulnicky & Skillman 1998), and more recently from RLs (Esteban et al. 2002;Peimbert 2003;Tsamis et al. 2003;Peimbert et al. 2005;López-Sánchez et al. 2007;Bresolin 2007). The C abundance determinations based on UV CELs are severely affected by uncertainties in the reddening correction. To further complicate the situation, the STIS spectrograph aboard the Hubble Space Telescope, the only instrument capable to detect the UV CELs of C in bright EHRs, stopped science operations in 2004, so that nowadays the observation of the optical CII RLs provides the only possibility for determining C abundances in EHRs. The study of the behavior of C/H and C/O ratios and their galactocentric gradients in galaxies of di...