The Zwicky Transient Facility: Surveys and Scheduler Bellm, Eric C.; Kulkarni, Shrinivas R.; Barlow, Tom ...
Publications of the Astronomical Society of the Pacific,
06/2019, Volume:
131, Issue:
1000
Journal Article
Peer reviewed
Open access
We present a novel algorithm for scheduling the observations of time-domain imaging surveys. Our integer linear programming approach optimizes an observing plan for an entire night by assigning ...targets to temporal blocks, enabling strict control of the number of exposures obtained per field and minimizing filter changes. A subsequent optimization step minimizes slew times between each observation. Our optimization metric self-consistently weights contributions from time-varying airmass, seeing, and sky brightness to maximize the transient discovery rate. We describe the implementation of this algorithm on the surveys of the Zwicky Transient Facility and present its on-sky performance.
Abstract
Using photometry collected with the Zwicky Transient Facility, we are conducting an ongoing survey for binary systems with short orbital periods (
with the goal of identifying new ...gravitational-wave sources detectable by the upcoming Laser Interferometer Space Antenna (LISA). We present a sample of 15 binary systems discovered thus far, with orbital periods ranging from 6.91 to 56.35 minutes. Of the 15 systems, seven are eclipsing systems that do not show signs of significant mass transfer. Additionally, we have discovered two AM Canum Venaticorum systems and six systems exhibiting primarily ellipsoidal variations in their lightcurves. We present follow-up spectroscopy and high-speed photometry confirming the nature of these systems, estimates of their LISA signal-to-noise ratios, and a discussion of their physical characteristics.
We report the discovery of the first short-period binary in which a hot subdwarf star (sdOB) filled its Roche lobe and started mass transfer to its companion. The object was discovered as part of a ...dedicated high-cadence survey of the Galactic plane named the Zwicky Transient Facility and exhibits a period of P = 39.3401(1) minutes, making it the most compact hot subdwarf binary currently known. Spectroscopic observations are consistent with an intermediate He-sdOB star with an effective temperature of = 42,400 300 K and a surface gravity of = 5.77 0.05. A high signal-to-noise ratio GTC+HiPERCAM light curve is dominated by the ellipsoidal deformation of the sdOB star and an eclipse of the sdOB by an accretion disk. We infer a low-mass hot subdwarf donor with a mass MsdOB = 0.337 0.015 and a white dwarf accretor with a mass MWD = 0.545 0.020 . Theoretical binary modeling indicates the hot subdwarf formed during a common envelope phase when a 2.5-2.8 star lost its envelope when crossing the Hertzsprung gap. To match its current , , , and masses, we estimate a post-common envelope period of 150 minutes and find that the sdOB star is currently undergoing hydrogen shell burning. We estimate that the hot subdwarf will become a white dwarf with a thick helium layer of 0.1 , merge with its carbon/oxygen white dwarf companion after 17 Myr, and presumably explode as a thermonuclear supernova or form an R CrB star.
Despite the fundamental importance of radical‐anion radical‐cation pairs in single‐electron transfer (SET) reactions, such species are still very rare and transient in nature. Since diborenes have ...highly electron‐rich BB double bonds, which makes them strong neutral reductants, we envisaged a possible realization of a boron‐centered radical‐anion radical‐cation pair by SET from a diborene to a borole species, which are known to form stable radical anions upon one‐electron reduction. However, since the reduction potentials of all know diborenes (E1/2=−1.05/−1.55 V) were not sufficiently negative to reduce MesBC4Ph4 (E1/2=−1.69 V), a suitable diborene, IiPr⋅(iPr)BB(iPr)⋅IiPr, was tailor‐made to comply with these requirements. With a halfwave potential of E1/2=−1.95 V, this diborene ranks amongst the most powerful neutral organic reductants known and readily reacted with MesBC4Ph4 by SET to afford a stable boron‐centered radical‐anion radical‐cation pair.
The tailor‐made diborene IiPr⋅(iPr)BB(iPr)⋅IiPr features an extremely electron‐rich B–B double bond making it one of the most powerful neutral organic reductants known to date (E1/2=−1.95 V). Its reducing power was successfully exploited in the synthesis of a very rare boron‐centered radical‐anion radical‐cation pair by one‐electron reduction of a borole, MesBC4Ph4. IiPr=1,3‐diisopropylimidazol‐2‐ylidene.
Most massive stars end their lives as red supergiants (RSGs), a short-lived evolutionary phase when they are known to pulsate with varying amplitudes. The RSG period-luminosity (PL) relation has been ...measured in the Milky Way, the Magellanic Clouds and M33 for about 120 stars in total. Using over 1500 epochs of R-band monitoring from the Palomar Transient Factory survey over a five-year period, we study the variability of 255 spectroscopically cataloged RSGs in M31. We find that all RGSs brighter than MK −10 mag (log(L/L ) > 4.8) are variable at ΔmR > 0.05 mag. Our period analysis finds 63 with significant pulsation periods. Using the periods found and the known values of MK for these stars, we derive the RSG PL relation in M31 and show that it is consistent with those derived earlier in other galaxies of different metallicities. We also detect, for the first time, a sequence of likely first-overtone pulsations. Comparison to stellar evolution models from MESA confirms the first-overtone hypothesis and indicates that the variable stars in this sample have 12 M < M < 24 M . As these RSGs are the immediate progenitors to Type II-P core-collapse supernovae (SNe), we also explore the implication of their variability in the initial-mass estimates for SN progenitors based on archival images of the progenitors. We find that this effect is small compared to the present measurement errors.
Non-iron [n]Metalloarenophanes Braunschweig, Holger; Kupfer, Thomas
Accounts of chemical research,
03/2010, Volume:
43, Issue:
3
Journal Article
Peer reviewed
The past 15 years have witnessed the creation of a wealth of fascinating applications for strained nferrocenophanes, in which the aromatic sandwich rings of ferrocene are covalently tethered (n ...denotes the number of bridging elements). By contrast, the related nmetalloarenophanes, that is, ansa-complexes not derived from ferrocene, have been neglected for a long time. In this Account, we present the tremendous progress that has been achieved in this field, mostly over the last five years. We focus on systems that have been developed in our laboratories, namely, those based on M(η6-C6H6)2 (where M is V, Cr, or Mo), Mn(η5-C5H5)(η6-C6H6), and M(η5-C5H5)(η7-C7H7) (where M is V or Cr). We begin by examining the synthetic precursors to nmetalloarenophanes, the selectively 1,1′-dilithiated sandwich complexes. These species can be isolated and characterized both in the solid state and in solution, and an appreciation of their structural properties is essential for the controlled synthesis of strained nmetalloarenophanes. About 25 different nmetalloarenophanes (n = 1, 2) have been obtained from 1,1′-dilithiated sandwich complexes by their stoichiometric reaction with appropriate element dihalides, and most have been fully characterized. X-ray diffraction data confirm the presence of tilted structures, with the extent of the tilt depending on the number of bridging elements, their covalent radii, and the nature of the metal center. The tilt angle, α, between the planes of the two carbocyclic ligands represents a good measure of the amount of ring strain present in these species, ranging from 31.23° for a highly strained 1boravanadoarenophane to 2.60° for the almost unstrained 2silatrochrocenophane. The strained character of nmetalloarenophanes is reflected in their rich and unusual reactivity. The thermodynamic driving force for most of the observed transformations is a significant reduction of molecular ring strain, as evidenced by smaller tilt angles in the products. For example, the 1sila derivatives undergo facile oxidative addition of the strained Si-areneC bond to low-valent transition metal complexes. Catalytic reaction with Karstedt’s catalyst results in the transition-metal-catalyzed ring-opening polymerization (ROP) of a 1silatrochrocenophane, yielding a polymeric species (M W = 6.4 × 103 g mol−1). The transition-metal-catalyzed ROP of a paramagnetic 1silavandoarenophane provides a rare example of a well-characterized macromolecule (M W ≥ 2.8 × 104 g mol−1) containing spin-active metal centers in the main chain. The BB bond of the 2borametalloarenophanes was found to be particularly susceptible to further functionalizations. Facile oxidative addition was observed, and the resulting 3diboraplatina derivatives could be successfully employed in the diboration of alkynes to form ansa-bis(boryl)alkenes. This transformation was also accomplished directly from the 2borametalloarenophanes, that is, a diboration of alkynes under both homogeneous and heterogeneous catalysis conditions. Similarly, the stoichiometric diboration of the NN double bond of azobenzene by a 3diboraplatinachromoarenophane is possible. The SiSi bond in 2silachromoarenophanes can also be used in further derivatizations, although higher temperatures are required. The bis-silylation of propyne proceeded via palladium mediation under homogeneous conditions to yield ansa-bis(silyl)alkenes. Finally, we discuss the electronic properties of nmetalloarenophanes developed in our laboratories, revealing correlations between selected NMR, EPR, and UV−visible parameters and molecular distortion.
Abstract
We present an analysis of new and archival data to the 20.506 minute LISA verification binary J052610.42+593445.32 (J0526+5934). Our joint spectroscopic and photometric analysis finds that ...the binary contains an unseen
M
1
= 0.89 ± 0.11
M
⊙
CO-core white dwarf primary with an
M
2
= 0.38 ± 0.07
M
⊙
post-core-burning subdwarf, or low-mass white dwarf, companion. Given the short orbital period and relatively large total binary mass, we find that LISA will detect this binary with signal-to-noise ratio 44 after 4 yr of observations. J0526+5934 is expected to merge within 1.8 ± 0.3 Myr and likely result in a D
6
scenario Type Ia supernova or form a He-rich star that will evolve into a massive single white dwarf.
The realization of a phosphine‐stabilized diborene, Et3P⋅(Mes)BB(Mes)⋅PEt3 (4), by KC8 reduction of Et3P⋅B2Mes2Br2 in benzene enabled the evaluation and comparison of its electronic structure to the ...previously described NHC‐stabilized diborene IMe⋅(Dur)BB(Dur)⋅IMe (1). Importantly, both species feature unusual electron‐rich boron centers. However, cyclic voltammetry, UV/Vis spectroscopy, and DFT calculations revealed a significant influence of the Lewis base on the reduction potential and absorption behavior of the BB double bond system. Thus, the stronger σ‐donor strength and larger electronegativity of the NHC ligand results in an energetically higher‐lying HOMO, making 1 a stronger neutral reductant as 4 (1: E1/2=−1.55 V; 4: −1.05 V), and a smaller HOMO–LUMO gap of 1 accompanied by a noticeable red‐shift of its lowest‐energy absorption band with respect to 4. Owing to the highly negative reduction potentials, 1 and 4 were easily oxidized to afford rare boron‐centered radical cations (5 and 6).
The NHC‐ and phosphine‐stabilized diborenes IMe⋅(Dur)BB(Dur)⋅IMe and PEt3⋅(Mes)BB(Mes)⋅PEt3 possess an unusually high boron‐centered electron density, making them very strong neutral reductants. Thus, removal of one electron from the BB π systems occurs readily by chemical oxidation with (C7H7)BArf4 to afford the corresponding radical cations. The nature of the Lewis base exerts strong influence on the electronic structure of the neutral and cationic diborene systems.
Abstract
We present the discovery of a new double-detonation progenitor system consisting of a hot subdwarf B (sdB) binary with a white dwarf companion with a
P
orb
= 76.34179(2) minutes orbital ...period. Spectroscopic observations are consistent with an sdB star during helium core burning residing on the extreme horizontal branch. Chimera light curves are dominated by ellipsoidal deformation of the sdB star and a weak eclipse of the companion white dwarf. Combining spectroscopic and light curve fits, we find a low-mass sdB star,
M
sdB
= 0.383 ± 0.028
M
⊙
with a massive white dwarf companion,
M
WD
= 0.725 ± 0.026
M
⊙
. From the eclipses we find a blackbody temperature for the white dwarf of 26,800 K resulting in a cooling age of ≈25 Myr whereas our
MESA
model predicts an sdB age of ≈170 Myr. We conclude that the sdB formed first through stable mass transfer followed by a common envelope which led to the formation of the white dwarf companion ≈25 Myr ago. Using the
MESA
stellar evolutionary code we find that the sdB star will start mass transfer in ≈6 Myr and in ≈60 Myr the white dwarf will reach a total mass of 0.92
M
⊙
with a thick helium layer of 0.17
M
⊙
. This will lead to a detonation that will likely destroy the white dwarf in a peculiar thermonuclear supernova. PTF1 J2238+7430 is only the second confirmed candidate for a double-detonation thermonuclear supernova. Using both systems we estimate that at least ≈1% of white dwarf thermonuclear supernovae originate from sdB+WD binaries with thick helium layers, consistent with the small number of observed peculiar thermonuclear explosions.
We present the discovery of the second binary with a Roche lobe-filling hot subdwarf transferring mass to a white dwarf (WD) companion. This 56 minute binary was discovered using data from the Zwicky ...Transient Facility. Spectroscopic observations reveal an He-sdOB star with an effective temperature of Teff = 33,700 1000 K and a surface gravity of log(g) = 5.54 0.11. The GTC+HiPERCAM light curve is dominated by the ellipsoidal deformation of the He-sdOB star and shows an eclipse of the He-sdOB by an accretion disk as well as a weak eclipse of the WD. We infer a He-sdOB mass of MsdOB = 0.41 0.04 M and a WD mass of MWD = 0.68 0.05 M . The weak eclipses imply a WD blackbody temperature of 63,000 10,000 K and a radius RWD = 0.0148 0.0020 R as expected for a WD of such high temperature. The He-sdOB star is likely undergoing hydrogen shell burning and will continue transferring mass for 1 Myr at a rate of 10−9 M yr−1, which is consistent with the high WD temperature. The hot subdwarf will then turn into a WD and the system will merge in 30 Myr. We suggest that Galactic reddening could bias discoveries toward preferentially finding Roche lobe-filling systems during the short-lived shell-burning phase. Studies using reddening-corrected samples should reveal a large population of helium core-burning hot subdwarfs with Teff 25,000 K in binaries of 60-90 minutes with WDs. Though not yet in contact, these binaries would eventually come into contact through gravitational-wave emission and explode as a subluminous thermonuclear supernova or evolve into a massive single WD.
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