Catching a star before explosion: the luminous blue variable progenitor of SN 2015bh

In this paper we analyse the pre-explosion spectrum of SN2015bh by performing radiative transfer simulations using the CMFGEN code. This object has attracted significant attention due to its remarkable similarity to SN2009ip in both its pre- and post-explosion behaviour. They seem to belong to a class of events for which the fate as a genuine core-collapse supernova or a non-terminal explosion is still under debate. Our CMFGEN models suggest that the progenitor of SN2015bh had an effective temperature between 8700 and 10000 K, luminosity in the range ~ 1.8-4.74e6 Lsun, contained at least 25% H in mass at the surface, and half-solar Fe abundances. The results also show that the progenitor of SN 2015bh generated an extended wind with a mass-loss rate of ~ 6e-4 to 1.5e-3 Msun/yr and a velocity of 1000 km/s. We determined that the wind extended to at least 2.57e14 cm and lasted for at least 30 days prior to the observations, releasing 5e-5 Msun into the circumstellar medium. In analogy to 2009ip, we propose that this is the material that the explosive ejecta could interact at late epochs, perhaps producing observable signatures that can be probed with future observations. We conclude that the progenitor of SN 2015bh was most likely a warm luminous blue variable of at least 35 Msun before the explosion. Considering the high wind velocity, we cannot exclude the possibility that the progenitor was a Wolf-Rayet star that inflated just before the 2013 eruption, similar to HD5980 during its 1994 episode. If the star survived, late-time spectroscopy may reveal either a similar LBV or a Wolf-Rayet star, depending on the mass of the H envelope before the explosion. If the star exploded as a genuine SN, 2015bh would be a remarkable case of a successful explosion after black-hole formation in a star with a possible minimum mass 35 Msun at the pre-SN stage.Comment: 13 pages, 10 figures, accepted for publication in A&

History and Options Regarding the Unfunded Liabilities of Alaska’s Public Employees’ and Teachers’ Retirement Systems

In early 2003, financial analysts working for the State of Alaska announced that the two largest public employee retirement systems in Alaska had become significantly underfunded.3 From fiscal year 2006 (July 1, 2005 through June 30, 2006) to date, the state has paid $6.951 billion— (an average of$534.7 million annually)—to pay down these obligations, which will be called “unfunded liabilities” in this paper.4 The State of Alaska has substantial unfunded liabilities remaining to pay off for these two systems, the Public Employees’ Retirement System (PERS) and the Teachers’ Retirement System (TRS). There is uncertainty about the size of these unfunded liabilities, and there are also different ways of calculating them. For example, the State of Alaska’s snapshot balance-sheet approach, subtracting the accrued liabilities from the assets, based on their actuarial value, produces an estimate of $6.609 billion for the combined unfunded liabilities of PERS and TRS.5 That figure is an estimate of the unfunded liabilities discounted to the present day. Estimates of the size of the unfunded liabilities particularly vary based on the use of different critical assumptions, such as the rate of future returns on investment. As an example, using an estimated rate of return of 2.142 percent instead of the State of Alaska’s assumption of 8 percent produces an estimate of$33.9 billion for the state’s unfunded liabilities. 6 The State of Alaska has committed to paying off the unfunded liabilities under a 25-year amortization schedule that started in 2014, so another highly relevant measurement of those liabilities appears to be the amount actuaries for the state currently project will be needed under that pay-off plan, which runs through fiscal year 2039. The state’s actuaries project that from fiscal year 2019 through fiscal year 2039 the state will pay a total of $10.815 billion in extra contributions—called “state assistance” or “additional state contributions” in this paper—to pay off the unfunded liabilities. 7 In contrast to the state’s snapshot estimate of$6.609 billion, this estimate of $10.815 billion in state assistance represents a flow of annual cash payments. That is, the$10.815 billion is an estimate of the total amount needed to eliminate the unfunded liabilities of PERS and TRS under the 25-year amortization schedule the state adopted in 2014. 4 Note that this state assistance is above and beyond the amount the state is projected to owe in its role as employer in the normal course of funding the two systems.8 Employers other than the state—primarily local governments and school districts—also participate in PERS and TRS, and the figure for state assistance covers not only unfunded liabilities attributed to the state but also a portion of the unfunded liabilities attributed to non-state employers. As explained more later, the state has assumed, by statute, the responsibility to pay for a share of the unfunded liability of these other employers. 9 This paper: • Describes the structure of the Alaska public employee retirement systems in the context of some unusual features of public employment on the Last Frontier • Reviews how the problem of unfunded liabilities came about • Examines how concerns over unfunded liabilities produced both changes and proposed changes in the retirement systems over the past dozen years, including proposals for changes in the allocation of burdens between the state and local governments in paying for retirement benefits • Describes current projections of future amounts needed to pay off the unfunded liabilities • Discusses how future estimates of the unfunded liabilities might change in response to economic and demographic factors • Discusses legal provisions protecting the rights of beneficiaries of the retirement systems • Lays out options for policymakers—other than the current policy of paying down the unfunded liabilities over time—including buyout, bailout, and bankruptcyNorthrim Bank University of Alaska Foundatio

The risk-adjusted performance of US buyouts

This paper assesses the risk-adjusted performance of US buyouts.risk-adjusted performance; US buyouts; risk; investment

Tuner: a tool for designing and optimizing ion optical systems

Designing and optimizing ion optical systems is often a complex and difficult task, which requires the use of computational tools to iterate and converge towards the desired characteristics and performances of the system. Very often these tools are not well adapted for exploring the numerous degrees of freedom, rendering the process long and tedious, as well as somewhat random due to the very large number of local minima typically found when looking for a particular optical solution. This paper presents a novel approach to finding the desired solution of an optical system, by providing the user with an instant feedback of the effects of changing parameters. The process of finding an approximate solution by manually adjusting parameters is greatly facilitated, at which point the final tune can be calculated by minimization according to a number of constraints.Comment: 15 pages, 6 figures, to be published in Nuclear Instruments and Methods

Is Eta Carinae a fast rotator, and how much does the companion influence the inner wind structure?

We analyze interferometric measurements of the Luminous Blue Variable Eta Carinae with the goal of constraining the rotational velocity of the primary star and probing the influence of the companion. Using 2-D radiative transfer models of latitude-dependent stellar winds, we find that prolate wind models with a ratio of the rotational velocity (vrot) to the critical velocity (vcrit) of W=0.77-0.92, inclination angle of i=60-90 degrees, and position angle PA=108-142 degrees reproduce simultaneously K-band continuum visibilities from VLTI/VINCI and closure phase measurements from VLTI/AMBER. Interestingly, oblate models with W=0.73-0.90 and i=80-90 degrees produce similar fits to the interferometric data, but require PA=210-230 degrees. Therefore, both prolate and oblate models suggest that the rotation axis of the primary star is not aligned with the Homunculus polar axis. We also compute radiative transfer models of the primary star allowing for the presence of a cavity and dense wind-wind interaction region created by the companion star. We find that the wind-wind interaction has a significant effect on the K-band image mainly via free-free emission from the compressed walls and, for reasonable model parameters, can reproduce the VLTI/VINCI visibilities taken at phase 0.92-0.93. We conclude that the density structure of the primary wind can be sufficiently disturbed by the companion, thus mimicking the effects of fast rotation in the interferometric observables. Therefore, fast rotation may not be the only explanation for the interferometric observations. Intense temporal monitoring and 3-D modeling are needed to resolve these issues.Comment: 6 pages, 4 color figures, accepted for publication in ApJ Letter