Aims.
We investigate electron temperature (
T
e
) and gas-phase oxygen abundance (
Z
Te
) measurements for galaxies in the local Universe (
z
< 0.25). Our sample comprises spectra from a total of ...264 emission-line systems, ranging from individual H
II
regions to whole galaxies, including 23 composite H
II
regions from star-forming main sequence galaxies in the MaNGA survey.
Methods.
We utilise 130 of these systems with directly measurable
T
e
(O
II
) to calibrate a new metallicity-dependent
T
e
(O
III
)–
T
e
(O
II
) relation that provides a better representation of our varied dataset than existing relations from the literature. We also provide an alternative
T
e
(O
III
)–
T
e
(N
II
) calibration. This new
T
e
method is then used to obtain accurate
Z
Te
estimates and form the mass – metallicity relation (MZR) for a sample of 118 local galaxies.
Results.
We find that all the
T
e
(O
III
)–
T
e
(O
II
) relations considered here systematically under-estimate
Z
Te
for low-ionisation systems by up to 0.6 dex. We determine that this is due to such systems having an intrinsically higher O
+
abundance than O
++
abundance, rendering
Z
Te
estimates based only on O
III
lines inaccurate. We therefore provide an empirical correction based on strong emission lines to account for this bias when using our new
T
e
(O
III
)–
T
e
(O
III
) and
T
e
(O
III
)–
T
e
(N
II
) relations. This allows for accurate metallicities (1
σ
= 0.08 dex) to be derived for any low-redshift system with an O
III
λ
4363 detection, regardless of its physical size or ionisation state. The MZR formed from our dataset is in very good agreement with those formed from direct measurements of metal recombination lines and blue supergiant absorption lines, in contrast to most other
T
e
-based and strong-line-based MZRs. Our new
T
e
method therefore provides an accurate and precise way of obtaining
Z
Te
for a large and diverse range of star-forming systems in the local Universe.
Early observations of Type Ia supernovae (SNe Ia) provide essential clues for understanding the progenitor system that gave rise to the terminal thermonuclear explosion. We present exquisite ...observations of SN 2019yvq, the second observed SN Ia, after iPTF 14atg, to display an early flash of emission in the ultraviolet (UV) and optical. Our analysis finds that SN 2019yvq was unusual, even when ignoring the initial flash, in that it was moderately underluminous for an SN Ia ( mag at peak) yet featured very high absorption velocities ( km s−1 for Si ii λ6355 at peak). We find that many of the observational features of SN 2019yvq, aside from the flash, can be explained if the explosive yield of radioactive 56Ni is relatively low (we measure ) and it and other iron-group elements are concentrated in the innermost layers of the ejecta. To explain both the UV/optical flash and peak properties of SN 2019yvq we consider four different models: interaction between the SN ejecta and a nondegenerate companion, extended clumps of 56Ni in the outer ejecta, a double-detonation explosion, and the violent merger of two white dwarfs. Each of these models has shortcomings when compared to the observations; it is clear additional tuning is required to better match SN 2019yvq. In closing, we predict that the nebular spectra of SN 2019yvq will feature either H or He emission, if the ejecta collided with a companion, strong Ca ii emission, if it was a double detonation, or narrow O i emission, if it was due to a violent merger.
ABSTRACT
We present the bolometric light curve, identification and analysis of the progenitor candidate, and preliminary modelling of AT 2016jbu (Gaia16cfr). We find a progenitor consistent with a ...∼ 22–25 M⊙ yellow hypergiant surrounded by a dusty circumstellar shell, in agreement with what has been previously reported. We see evidence for significant photometric variability in the progenitor, as well as strong Hα emission consistent with pre-existing circumstellar material. The age of the environment, as well as the resolved stellar population surrounding AT 2016jbu, supports a progenitor age of >10 Myr, consistent with a progenitor mass of ∼22 M⊙. A joint analysis of the velocity evolution of AT 2016jbu and the photospheric radius inferred from the bolometric light curve shows the transient is consistent with two successive outbursts/explosions. The first outburst ejected material with velocity ∼650 km s−1, while the second, more energetic event ejected material at ∼4500 km s−1. Whether the latter is the core collapse of the progenitor remains uncertain. We place a limit on the ejected 56Ni mass of <0.016 M⊙. Using the Binary Population And Spectral Synthesis (BPASS) code, we explore a wide range of possible progenitor systems and find that the majority of these are in binaries, some of which are undergoing mass transfer or common-envelope evolution immediately prior to explosion. Finally, we use the SuperNova Explosion Code (SNEC) to demonstrate that the low-energy explosions within some of these binary systems, together with sufficient circumstellar material, can reproduce the overall morphology of the light curve of AT 2016jbu.
ABSTRACT
We present the results from a high-cadence, multiwavelength observation campaign of AT 2016jbu (aka Gaia16cfr), an interacting transient. This data set complements the current literature by ...adding higher cadence as well as extended coverage of the light-curve evolution and late-time spectroscopic evolution. Photometric coverage reveals that AT 2016jbu underwent significant photometric variability followed by two luminous events, the latter of which reached an absolute magnitude of MV ∼ −18.5 mag. This is similar to the transient SN 2009ip whose nature is still debated. Spectra are dominated by narrow emission lines and show a blue continuum during the peak of the second event. AT 2016jbu shows signatures of a complex, non-homogeneous circumstellar material (CSM). We see slowly evolving asymmetric hydrogen line profiles, with velocities of 500 km s−1 seen in narrow emission features from a slow-moving CSM, and up to 10 000 km s−1 seen in broad absorption from some high-velocity material. Late-time spectra (∼+1 yr) show a lack of forbidden emission lines expected from a core-collapse supernova and are dominated by strong emission from H, He i, and Ca ii. Strong asymmetric emission features, a bumpy light curve, and continually evolving spectra suggest an inhibit nebular phase. We compare the evolution of H α among SN 2009ip-like transients and find possible evidence for orientation angle effects. The light-curve evolution of AT 2016jbu suggests similar, but not identical, circumstellar environments to other SN 2009ip-like transients.
We present the first uniform treatment of long duration gamma-ray burst (GRB) host galaxy detections and upper limits over the redshift range 3 < z < 5, a key epoch for observational and theoretical ...efforts to understand the processes, environments, and consequences of early cosmic star formation (SF). We contribute deep imaging observations of 13 GRB positions yielding the discovery of 8 new host galaxies. We use this data set in tandem with previously published observations of 31 further GRB positions to estimate or constrain the host galaxy rest-frame ultraviolet (UV; lambda = 1600 Angstrom) absolute magnitudes M sub(UV). We then use the combined set of 44 M sub(UV) estimates and limits to construct the M sub(UV) luminosity function (LF) for GRB host galaxies over 3 < z < 5 and compare it to expectations from Lyman break galaxy (LBG) photometric surveys with the Hubble Space Telescope. Adopting standard prescriptions for the luminosity dependence of galaxy dust obscuration (and hence, total SF rate), we find that our LF is compatible with LBG observations over a factor of 600 x in host luminosity, from M sub(UV) = -22.5 mag to >-15.6 mag, and with extrapolations of the assumed Schechter-type LF well beyond this range. We review proposed astrophysical and observational biases for our sample, and find that they are for the most part minimal. We therefore conclude, as the simplest interpretation of our results, that GRBs successfully trace UV metrics of cosmic SF over the range 3 < z < 5. Our findings suggest that GRBs provide an accurate picture of star formation processes from z approx = 3 out to the highest redshifts.
We present observations of supernova (SN) 2017ens, discovered by the ATLAS survey and identified as a hot blue object through the GREAT program. The redshift z = 0.1086 implies a peak brightness of ...Mg = −21.1 mag, placing the object within the regime of superluminous supernovae. We observe a dramatic spectral evolution, from initially being blue and featureless, to later developing features similar to those of the broadlined Type Ic SN 1998bw, and finally showing ∼2000 km s−1 wide H and Hβ emission. Relatively narrow Balmer emission (reminiscent of a SN IIn) is present at all times. We also detect coronal lines, indicative of a dense circumstellar medium. We constrain the progenitor wind velocity to ∼50-60 km s−1 based on P-Cygni profiles, which is far slower than those present in Wolf-Rayet stars. This may suggest that the progenitor passed through a luminous blue variable phase, or that the wind is instead from a binary companion red supergiant star. At late times we see the ∼2000 km s−1 wide H emission persisting at high luminosity (∼3 × 1040 erg s−1) for at least 100 day, perhaps indicative of additional mass loss at high velocities that could have been ejected by a pulsational pair instability.
Over the last decade there has been immense progress in the follow-up of short and long gamma-ray bursts (GRBs), resulting in a significant rise in the detection rate of X-ray and optical afterglows, ...in the determination of GRB redshifts, and of the identification of the underlying host galaxies. Nevertheless, our theoretical understanding of the progenitors and central engines powering these vast explosions is lagging behind, and a newly identified class of ultra-long GRBs has fuelled speculation on the existence of a new channel of GRB formation. In this paper we present high signal-to-noise X-Shooter observations of the host galaxy of GRB 130925A, which is the fourth unambiguously identified ultra-long GRB, with prompt γ-ray emission detected for ~20 ks. The GRB line of sight was close to the host galaxy nucleus, and our spectroscopic observations cover this region along the bulge/disk of the galaxy and a bright star-forming region within the outskirts of the galaxy. From our broad wavelength coverage, we obtain accurate metallicity and dust-extinction measurements at the galaxy nucleus and at an outer star-forming region, and measure a super-solar metallicity at both locations, placing this galaxy within the 10–20% most metal-rich GRB host galaxies. Such a high metal enrichment has significant implications on the progenitor models of both long and ultra-long GRBs, although the edge-on orientation of the host galaxy does not allow us to rule out a large metallicity variation along our line of sight. The spatially resolved spectroscopic observations presented in this paper offer important insight into variations in the metal and dust abundance within GRB host galaxies. However, they also illustrate the need for integral field unit observations on a larger sample of GRB host galaxies of a variety of metallicities to provide a more quantitative view on the relation between the GRB circumburst environment and the galaxy-whole properties.
Context. Short-duration gamma-ray bursts (GRBs) with extended emission form a subclass of short GRBs, comprising about 15% of the short-duration sample. Afterglow detections of short GRBs are also ...rare (about 30%) because of their lower luminosity. Aims. We present a multiband data set of the short burst with extended emission, GRB 150424A, comprising of GROND observations, complemented with data from Swift/UVOT, Swift/XRT, HST, Keck/LRIS, and data points from the literature. The GRB 150424A afterglow shows an extended plateau phase, lasting about 8 h. The analysis of this unique GRB afterglow might shed light on the understanding of afterglow plateau emission, the nature of which is still under debate. Methods. We present a phenomenological analysis made by applying fireball closure relations and interpret the findings in the context of the fireball model. We discuss the plausibility of a magnetar as a central engine, which would be responsible for additional and prolonged energy injection into the fireball. Results. We find convincing evidence for energy injection into the afterglow of GRB 150424A. We find that a magnetar spin-down as the source for a prolonged energy injection requires that at least 4% of the spin-down energy is converted into radiation.
Abstract
The ultraviolet (UV) extinction feature at 2175 Å is ubiquitously observed in the Galaxy but is rarely detected at high redshifts. Here we report the spectroscopic detection of the 2175 Å ...bump on the sightline to the
γ
-ray burst (GRB) afterglow GRB 180325A at
z
= 2.2486, the only unambiguous detection over the past 10 years of GRB follow-up, at four different epochs with the Nordic Optical Telescope (NOT) and the Very Large Telescope (VLT)/X-shooter. Additional photometric observations of the afterglow are obtained with the Gamma-Ray burst Optical and Near-Infrared Detector (GROND). We construct the near-infrared to X-ray spectral energy distributions (SEDs) at four spectroscopic epochs. The SEDs are well described by a single power law and an extinction law with
R
V
≈ 4.4,
A
V
≈ 1.5, and the 2175 Å extinction feature. The bump strength and extinction curve are shallower than the average Galactic extinction curve. We determine a metallicity of Zn/H > −0.98 from the VLT/X-shooter spectrum. We detect strong neutral carbon associated with the GRB with equivalent width of
W
r
(
λ
1656) = 0.85 ± 0.05. We also detect optical emission lines from the host galaxy. Based on the H
α
emission-line flux, the derived dust-corrected star formation rate is ∼46 ± 4
M
⊙
yr
−1
and the predicted stellar mass is log
M
*
/
M
⊙
∼ 9.3 ± 0.4, suggesting that the host galaxy is among the main-sequence star-forming galaxies.
We present the results of our photometric and spectroscopic follow-up of the intermediate-luminosity optical transient AT 2017jfs. At peak, the object reaches an absolute magnitude of Mg = −15.46 ± ...0.15 mag and a bolometric luminosity of 5.5 × 1041 erg s−1. Its light curve has the double-peak shape typical of luminous red novae (LRNe), with a narrow first peak bright in the blue bands, while the second peak is longer-lasting and more luminous in the red and near-infrared (NIR) bands. During the first peak, the spectrum shows a blue continuum with narrow emission lines of H and Fe II. During the second peak, the spectrum becomes cooler, resembling that of a K-type star, and the emission lines are replaced by a forest of narrow lines in absorption. About 5 months later, while the optical light curves are characterized by a fast linear decline, the NIR ones show a moderate rebrightening, observed until the transient disappears in solar conjunction. At these late epochs, the spectrum becomes reminiscent of that of M-type stars, with prominent molecular absorption bands. The late-time properties suggest the formation of some dust in the expanding common envelope or an IR echo from foreground pre-existing dust. We propose that the object is a common-envelope transient, possibly the outcome of a merging event in a massive binary, similar to NGC 4490−2011OT1.
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