Does the creation of a boreal hydroelectric reservoir result in a net change in evaporation?

AbstractEstimates of water consumption from hydroelectricity production are hampered by a lack of common methodological approaches. Studies typically use gross evaporation estimates which do not take into account the evaporative water loss from the pre-flooded ecosystems that would occur without the presence of a reservoir. We evaluate the net change in evaporation following the creation of a hydroelectric reservoir located in the Canadian boreal region. We use a direct measurement technique (eddy covariance) over four different ecosystems to evaluate the pre- and post-flood landscape water flux over a five-year period. The net effect of reservoir creation was to increase evaporation over that of the pre-flooded ecosystem. This change was dependent both on management and differences in the timing of the evaporation with nighttime and autumn contributing strongly to the reservoir evaporation. Managed reduction of water level, and thus the evaporating area, reduced the evaporation

Massive stars exploding in a He-rich circumstellar medium. VI. Observations of two distant Type Ibn supernova candidates discovered by La Silla-QUEST

We present optical observations of the peculiar stripped-envelope supernovae (SNe) LSQ12btw and LSQ13ccw discovered by the La Silla-QUEST survey. LSQ12btw reaches an absolute peak magnitude of M(g) = -19.3 +- 0.2, and shows an asymmetric light curve. Stringent prediscovery limits constrain its rise time to maximum light to less than 4 days, with a slower post-peak luminosity decline, similar to that experienced by the prototypical SN~Ibn 2006jc. LSQ13ccw is somewhat different: while it also exhibits a very fast rise to maximum, it reaches a fainter absolute peak magnitude (M(g) = -18.4 +- 0.2), and experiences an extremely rapid post-peak decline similar to that observed in the peculiar SN~Ib 2002bj. A stringent prediscovery limit and an early marginal detection of LSQ13ccw allow us to determine the explosion time with an uncertainty of 1 day. The spectra of LSQ12btw show the typical narrow He~I emission lines characterising Type Ibn SNe, suggesting that the SN ejecta are interacting with He-rich circumstellar material. The He I lines in the spectra of LSQ13ccw exhibit weak narrow emissions superposed on broad components. An unresolved Halpha line is also detected, suggesting a tentative Type Ibn/IIn classification. As for other SNe~Ibn, we argue that LSQ12btw and LSQ13ccw likely result from the explosions of Wolf-Rayet stars that experienced instability phases prior to core collapse. We inspect the host galaxies of SNe Ibn, and we show that all of them but one are hosted in spiral galaxies, likely in environments spanning a wide metallicity range.Comment: 15 pages, 9 figures, 4 tables. Accepted by MNRA

Don't Blink: Constraining the Circumstellar Environment of the Interacting Type Ia Supernova 2015cp

Despite their cosmological utility, the progenitors of Type Ia supernovae (SNe Ia) are still unknown, with many efforts focused on whether accretion from a nondegenerate companion can grow a carbon-oxygen white dwarf to near the Chandrasekhar mass. The association of SNe Ia resembling SN 1991T ("91T-like") with circumstellar interaction may be evidence for this "single-degenerate" channel. However, the observed circumstellar medium (CSM) in these interacting systems is unlike a stellar wind -- of particular interest, it is sometimes detached from the stellar surface, residing at $\sim 10^{16}~{\rm cm}$. A Hubble Space Telescope (HST) program to discover detached CSM around 91T-like SNe Ia successfully discovered interaction nearly two years after explosion in SN 2015cp (Graham et al., 2018). In this work, we present radio and X-ray follow-up observations of SN 2015cp and analyze them in the framework of Harris, Nugent, & Kasen (2016) to limit the properties of a constant-density CSM shell in this system. Assuming the HST detection was shortly after the shock crossed the CSM, we constrain the total CSM mass in this system to be $< 0.5~{\rm M_\odot}$. This limit is comparable to the CSM mass of supernova PTF11kx, but does not rule out lower masses predicted for recurrent novae. From lessons learned modeling PTF11kx and SN 2015cp, we suggest a strategy for future observations of these events to increase the sample of known interacting SNe Ia.Comment: 13 pages, 5 figures, published in The Astrophysical Journa

Improved Standardization of Type II-P Supernovae: Application to an Expanded Sample

In the epoch of precise and accurate cosmology, cross-confirmation using a variety of cosmographic methods is paramount to circumvent systematic uncertainties. Owing to progenitor histories and explosion physics differing from those of Type Ia SNe (SNe Ia), Type II-plateau supernovae (SNe II-P) are unlikely to be affected by evolution in the same way. Based on a new analysis of 17 SNe II-P, and on an improved methodology, we find that SNe II-P are good standardizable candles, almost comparable to SNe Ia. We derive a tight Hubble diagram with a dispersion of 10% in distance, using the simple correlation between luminosity and photospheric velocity introduced by Hamuy & Pinto 2002. We show that the descendent method of Nugent et al. 2006 can be further simplified and that the correction for dust extinction has low statistical impact. We find that our SN sample favors, on average, a very steep dust law with total to selective extinction R_V<2. Such an extinction law has been recently inferred for many SNe Ia. Our results indicate that a distance measurement can be obtained with a single spectrum of a SN II-P during the plateau phase combined with sparse photometric measurements.Comment: ApJ accepted version. Minor change

Keck Observations of the Young Metal-Poor Host Galaxy of the Super-Chandrasekhar-Mass Type Ia Supernova SN 2007if

We present Keck LRIS spectroscopy and $g$-band photometry of the metal-poor, low-luminosity host galaxy of the super-Chandrasekhar mass Type Ia supernova SN 2007if. Deep imaging of the host reveals its apparent magnitude to be $m_g=23.15\pm0.06$, which at the spectroscopically-measured redshift of $z_{helio}=0.07450\pm0.00015$ corresponds to an absolute magnitude of $M_g=-14.45\pm0.06$. Galaxy $g-r$ color constrains the mass-to-light ratio, giving a host stellar mass estimate of $\log(M_*/M_\odot)=7.32\pm0.17$. Balmer absorption in the stellar continuum, along with the strength of the 4000\AA\ break, constrain the age of the dominant starburst in the galaxy to be $t_\mathrm{burst}=123^{+165}_{-77}$ Myr, corresponding to a main-sequence turn-off mass of $M/M_\odot=4.6^{+2.6}_{-1.4}$. Using the R$_{23}$ method of calculating metallicity from the fluxes of strong emission lines, we determine the host oxygen abundance to be $12+\log(O/H)_\mathrm{KK04}=8.01\pm0.09$, significantly lower than any previously reported spectroscopically-measured Type Ia supernova host galaxy metallicity. Our data show that SN 2007if is very likely to have originated from a young, metal-poor progenitor.Comment: 15 pages, 9 figures; accepted for publication in Ap

Hubble Space Telescope studies of low-redshift Type Ia supernovae: Evolution with redshift and ultraviolet spectral trends

We present an analysis of the maximum light, near ultraviolet (NUV; 2900-5500 A) spectra of 32 low redshift (0.001<z<0.08) Type Ia supernovae (SNe Ia), obtained with the Hubble Space Telescope (HST). We combine this spectroscopic sample with high-quality gri light curves obtained with robotic telescopes to measure photometric parameters, such as stretch, optical colour, and brightness. By comparing our data to a comparable sample of SNe Ia at intermediate-z (0.4<z<0.9), we detect modest spectral evolution (3-sigma), in the sense that our mean low-z NUV spectrum has a depressed flux compared to its intermediate-z counterpart. We also see a strongly increased dispersion about the mean with decreasing wavelength, confirming the results of earlier surveys. These trends are consistent with changes in metallicity as predicted by contemporary SN Ia spectral models. We also examine the properties of various NUV spectral diagnostics in the individual spectra. We find a general correlation between stretch and the velocity (or position) of many NUV spectral features. In particular, we observe that higher stretch SNe have larger Ca II H&K velocities, that also correlate with host galaxy stellar mass. This latter trend is probably driven by the well-established correlation between stretch and stellar mass. We find no trends between UV spectral features and optical colour. Mean spectra constructed according to whether the SN has a positive or negative Hubble residual show very little difference at NUV wavelengths, indicating that the NUV evolution and variation we identify do not directly correlate with Hubble residuals. Our work confirms and strengthens earlier conclusions regarding the complex behaviour of SNe Ia in the NUV spectral region, but suggests the correlations we find are more useful in constraining progenitor models than improving the use of SNe Ia as cosmological probes.Comment: 22 pages, 14 figures, accepted in MNRAS with minor changes - Spectra are available on WISeREP, http://www.weizmann.ac.il/astrophysics/wiserep

Discovery and Follow-up Observations of the Young Type Ia Supernova 2016coj

The Type~Ia supernova (SN~Ia) 2016coj in NGC 4125 (redshift $z=0.004523$) was discovered by the Lick Observatory Supernova Search 4.9 days after the fitted first-light time (FFLT; 11.1 days before $B$-band maximum). Our first detection (pre-discovery) is merely $0.6\pm0.5$ day after the FFLT, making SN 2016coj one of the earliest known detections of a SN Ia. A spectrum was taken only 3.7 hr after discovery (5.0 days after the FFLT) and classified as a normal SN Ia. We performed high-quality photometry, low- and high-resolution spectroscopy, and spectropolarimetry, finding that SN 2016coj is a spectroscopically normal SN Ia, but with a high velocity of \ion{Si}{2} $\lambda$6355 ($\sim 12,600$\,\kms\ around peak brightness). The \ion{Si}{2} $\lambda$6355 velocity evolution can be well fit by a broken-power-law function for up to a month after the FFLT. SN 2016coj has a normal peak luminosity ($M_B \approx -18.9 \pm 0.2$ mag), and it reaches a $B$-band maximum \about16.0~d after the FFLT. We estimate there to be low host-galaxy extinction based on the absence of Na~I~D absorption lines in our low- and high-resolution spectra. The spectropolarimetric data exhibit weak polarization in the continuum, but the \ion{Si}{2} line polarization is quite strong ($\sim 0.9\% \pm 0.1\%$) at peak brightness.Comment: Submitte

A Mismatch in the Ultraviolet Spectra between Low-Redshift and Intermediate-Redshift Type Ia Supernovae as a Possible Systematic Uncertainty for Supernova Cosmology

We present Keck high-quality rest-frame ultraviolet (UV) through optical spectra of 21 Type Ia supernovae (SNe Ia) in the redshift range 0.11 < z < 0.37 and a mean redshift of 0.22 that were discovered during the Sloan Digital Sky Survey-II (SDSS-II) SN Survey. Using the broad-band photometry of the SDSS survey, we are able to reconstruct the SN host-galaxy spectral energy distributions (SEDs), allowing for a correction for the host-galaxy contamination in the SN Ia spectra. Comparison of composite spectra constructed from a subsample of 17 high-quality spectra to those created from a low-redshift sample with otherwise similar properties shows that the Keck/SDSS SNe Ia have, on average, extremely similar rest-frame optical spectra but show a UV flux excess. This observation is confirmed by comparing synthesized broad-band colors of the individual spectra, showing a difference in mean colors at the 2.4 - 4.4 sigma level for various UV colors. We further see a slight difference in the UV spectral shape between SNe with low-mass and high-mass host galaxies. Additionally, we detect a relationship between the flux ratio at 2770 and 2900 A and peak luminosity that differs from that observed at low redshift. We find that changing the UV SED of an SN Ia within the observed dispersion can change the inferred distance moduli by ~0.1 mag. This effect only occurs when the data probe the rest-frame UV. We suggest that this discrepancy could be due to differences in the host-galaxy population of the two SN samples or to small-sample statistics.Comment: 28 pages, 21 figures, accepted by AJ, spectra are available at http://www.cfa.harvard.edu/~rfoley/data

The Spectroscopic Diversity of Type Ia Supernovae

We present 2603 spectra of 462 nearby Type Ia supernovae (SN Ia) obtained during 1993-2008 through the Center for Astrophysics Supernova Program. Most of the spectra were obtained with the FAST spectrograph at the FLWO 1.5m telescope and reduced in a consistent manner, making data set well suited for studies of SN Ia spectroscopic diversity. We study the spectroscopic and photometric properties of SN Ia as a function of spectroscopic class using the classification schemes of Branch et al. and Wang et al. The width-luminosity relation appears to be steeper for SN Ia with broader lines. Based on the evolution of the characteristic Si II 6355 line, we propose improved methods for measuring velocity gradients, revealing a larger range than previously suspected, from ~0 to ~400 km/s/day considering the instantaneous velocity decline rate at maximum light. We find a weaker and less significant correlation between Si II velocity and intrinsic B-V color at maximum light than reported by Foley et al., owing to a more comprehensive treatment of uncertainties and host galaxy dust. We study the extent of nuclear burning and report new detections of C II 6580 in 23 early-time spectra. The frequency of C II detections is not higher in SN Ia with bluer colors or narrower light curves, in conflict with the recent results of Thomas et al. Based on nebular spectra of 27 SN Ia, we find no relation between the FWHM of the iron emission feature at ~4700 A and Dm15(B) after removing the two low-luminosity SN 1986G and SN 1991bg, suggesting that the peak luminosity is not strongly dependent on the kinetic energy of the explosion for most SN Ia. Finally, we confirm the correlation of velocity shifts in some nebular lines with the intrinsic B-V color of SN Ia at maximum light, although several outliers suggest a possible non-monotonic behavior for the largest blueshifts.Comment: 36 pages (emulateapj), 23 figures. Accepted for publication in AJ. Spectroscopic data available at http://www.cfa.harvard.edu/supernova/SNarchive.html . New SNID template set available at http://marwww.in2p3.fr/~blondin/software/snid/index.html . Minor changes from v1 to conform to published versio

Confined dense circumstellar material surrounding a regular type II supernova

With the advent of new wide-field, high-cadence optical transient surveys, our understanding of the diversity of core-collapse supernovae has grown tremendously in the last decade. However, the pre-supernova evolution of massive stars, which sets the physical backdrop to these violent events, is theoretically not well understood and difficult to probe observationally. Here we report the discovery of the supernova iPTF 13dqy = SN 2013fs a mere ∼3 h after explosion. Our rapid follow-up observations, which include multiwavelength photometry and extremely early (beginning at ∼6 h post-explosion) spectra, map the distribution of material in the immediate environment (≲1015 cm) of the exploding star and establish that it was surrounded by circumstellar material (CSM) that was ejected during the final ∼1 yr prior to explosion at a high rate, around 10-3 solar masses per year. The complete disappearance of flash-ionized emission lines within the first several days requires that the dense CSM be confined to within ≲1015 cm, consistent with radio non-detections at 70–100 days. The observations indicate that iPTF 13dqy was a regular type II supernova; thus, the finding that the probable red supergiant progenitor of this common explosion ejected material at a highly elevated rate just prior to its demise suggests that pre-supernova instabilities may be common among exploding massive stars. © 2017 Nature Publishing Grou