The
Polstar
mission will provide a space-borne 60 cm spectropolarimeter operating at ultraviolet (UV) wavelengths, capturing all four Stokes parameters (intensity, two linear polarization components, ...and circular polarization).
Polstar
’s capabilities are designed to meet its goal of determining how circumstellar gas flows alter and inform massive star evolution, affect the stellar remnant population, and stir and enrich the interstellar medium (ISM). These will be achieved by investigating the dynamical geometries in the winds and disks of hot stars, the composition and magnetic alignment of interstellar dust, and the star-forming accretion disks of UV-bright stars at an important transition boundary. Together these areas map out a kind of two-way interface between massive stars and their effect on our galaxy, wherein the stellar winds enrich the ISM with metals and kinetic energy, preconditioning their environment and the stellar endpoints prior to undergoing supernova. The ISM dust in turn reveals the composition and magnetic environment leading to new star formation, and the accretion disks of Herbig Ae/Be stars reveal how the ISM gas returns to make new massive stars.
Polstar
will combine high-resolution spectroscopy in the time domain with high-precision UV polarimetry. Doppler-shifted UV resonance line opacity will provide information about circumstellar kinematics, while polarization gives complementary geometric information about unseen structures. The composition and magnetic alignment of the smallest interstellar dust grains provides a probe of the ISM utilizing radiative alignment theory (RAT).
Polstar
will operate in the far-UV (FUV) at 122–200 nm at high spectral resolution of around
R
∼
30
k
, and at FUV and near-UV (NUV) wavelengths of 122–320 nm at lower spectral resolutions of
0.1
−
1
k
. Detection of polarization levels as weak as 0.1% are expected, with a temporal cadence ranging from 5–10 minutes for most wind variability studies, to hours or days for sampling rotation, to days or weeks for sampling binary orbits, to months to a year for sampling substructure in the inner regions of protoplanetary disks. Sub-meter-class aperture is well suited to access this wide array of time domain science, made possible by restricting to a few hundred bright, massive stars, necessarily extincted by a small to moderate column of interstellar dust, informing both the attributes of the stars and the ISM through which they are seen. As such, the focus is on our own galaxy and its evolutionary drivers, but a few targets in the Magellanic clouds offer the potential to extend this understanding to low-metallicity environments.
Polstar is a proposed NASA MIDEX mission that carries a high resolution UV spectropolarimeter capable of measure all four Stokes parameters onboard a 60 cm telescope. The mission has been designed to ...pioneer the field of time-domain UV spectropolarimetry. Time domain UV spectropolarimetry offers the best resource to determine the geometry and physical conditions of protoplanetary disks from the stellar surface to <5 AU.We detail two key objectives that a dedicated time domain UV
spectropolarimetry survey, such as that enabled by Polstar or a similar mission concept, could achieve: 1) Test the hypothesis that magneto-accretion operating in young planet-forming disks around lower-mass stars transitions to boundary layer accretion
in planet-forming disks around higher mass stars; and 2) Discriminate whether transient events in the innermost regions of planet-forming disks of intermediate mass stars are caused by inner disk misalignments or from stellar or disk emissions.
Using data acquired as part of a unique Hubble Heritage imaging program of broadband colors of the interacting spiral system M51/NGC 5195, we have conducted a photometric study of the stellar ...associations across the entire disk of the galaxy in order to assess trends in size, luminosity, and local environment associated with the recent star formation (SF) activity in the system. Starting with a sample of over 900 potential associations, we have produced color-magnitude and color-color diagrams for the 120 associations that were deemed to be single-aged. It has been found that main-sequence (MS) turnoffs are not evident for the vast majority of the stellar associations in our set, potentially due to the overlap of isochronal tracks at the high mass end of the MS, and the limited depth of our images at the distance of M51. In order to obtain ages for more of our sample, we produced model spectral energy distributions (SEDs) to fit to the data from the GALEXEV simple stellar population models of Bruzual & Charlot. These SEDs can be used to determine age, size, mass, metallicity, and dust content of each association via a simple Delta *y2 minimization to each association's B-, V-, and I-band fluxes. The derived association properties are mapped as a function of location, and recent trends in SF history of the galaxy are explored in light of these results. This work is the first phase in a program that will compare these stellar systems with their environments using ultraviolet data from the Galaxy Evolution Explorer and infrared data from Spitzer, and ultimately we plan to apply the same stellar population mapping methodology to other nearby face-on spiral galaxies.
The most massive stars are thought to lose a significant fraction of their mass in a steady wind during the main-sequence and blue supergiant phases. This in turn sets the stage for their further ...evolution and eventual supernova, and preconditions the surrounding medium for all following events, with consequences for ISM energization, chemical enrichment, and dust formation. Understanding these processes requires accurate observational constraints on the mass-loss rates of the most luminous stars, which can also be used to test theories of stellar wind driving. In the past, mass-loss rates have been characterized via collisional emission processes such as optical Hα and free-free radio emission, but these so-called “density squared” diagnostics require correction in the presence of widespread clumping. Recent observational and theoretical evidence points to the likelihood of a ubiquitously high level of such clumping in hot-star winds, but quantifying its effects requires a deeper understanding of the complex dynamics of radiatively driven winds and their stochastic instabilities. Furthermore, large-scale structures initiating in surface anisotropies and propagating throughout the wind can also affect wind driving and alter mass-loss diagnostics. Time series spectroscopy of high resonance-line opacity in the UV, capable of high resolution and high signal-to-noise, are required to better understand these complex dynamics, and more accurately determine mass-loss rates. The proposed Polstar mission (Scowen et al. 2022, this volume) provides the necessary resolution at the Sobolev (∼10 km s−1) or sound-speed (∼20 km s−1) scale, for over three dozen bright galactic massive stars with signal-to noise an order of magnitude above that of the celebrated MEGA campaign (Massa et al. 1995) of the International Ultraviolet Explorer (IUE), via continuous observations that track propagating structures through the winds in real time. Supporting geometric constraints are provided by the polarimetric capabilities present in all the datasets of such a mission.
The current consensus is that at least half of the OB stars are formed in binary or multiple star systems. The evolution of OB stars is greatly influenced by whether the stars begin as close ...binaries, and the evolution of the binary systems depend on whether the mass transfer is conservative or nonconservative. FUV/NUV spectropolarimetry is poised to answer the latter question. This paper discusses how the Polstar spectropolarimetry mission can characterize the degree of nonconservative mass transfer that occurs at various stages of binary evolution, from the initial mass reversal to the late Algol phase, and quantify its amount. The proposed instrument combines spectroscopic and polarimetric capabilities, where the spectroscopy can resolve Doppler shifts in UV resonance lines with 10 km/s precision, and polarimetry can resolve linear polarization with 10−3 precision or better. The spectroscopy will identify absorption by mass streams and other plasmas seen in projection against the stellar disk as a function of orbital phase, as well as scattering from extended splash structures, including jets. The polarimetry tracks the light coming from material not seen against the stellar disk, allowing the geometry of the scattering to be tracked, resolving ambiguities left by the spectroscopy and light-curve information. For example, nonconservative mass streams ejected in the polar direction will produce polarization of the opposite sign from conservative transfer accreting in the orbital plane. Time domain coverage over a range of phases of the binary orbit are well supported by the Polstar observing strategy. Special attention will be given to the epochs of enhanced systemic mass loss that have been identified from IUE observations (pre-mass reversal and tangential gas stream impact). We show how the history of systemic mass and angular momentum loss/gain episodes can be inferred via ensemble evolution through the r–q diagram. Combining the above elements will significantly improve our understanding of the mass transfer process and the amount of mass that can escape from the system, an important channel for changing the final mass and ultimate supernova of a large number of massive stars found in binaries at close enough separation to undergo interaction.
The winds of massive stars are important for their direct impact on the interstellar medium, and for their influence on the final state of a star prior to it exploding as a supernova. However, the ...dynamics of these winds is understood primarily via their illumination from a single central source. The Doppler shift seen in resonance lines is a useful tool for inferring these dynamics, but the mapping from that Doppler shift to the radial distance from the source is ambiguous. Binary systems can reduce this ambiguity by providing a second light source at a known radius in the wind, seen from orbitally modulated directions. From the nature of the collision between the winds, a massive companion also provides unique additional information about wind momentum fluxes. Since massive stars are strong ultraviolet (UV) sources, and UV resonance line opacity in the wind is strong, UV instruments with a high resolution spectroscopic capability are essential for extracting this dynamical information. Polarimetric capability also helps to further resolve ambiguities in aspects of the wind geometry that are not axisymmetric about the line of sight, because of its unique access to scattering direction information. We review how the proposed MIDEX-scale mission Polstar can use UV spectropolarimetric observations to critically constrain the physics of colliding winds, and hence radiatively-driven winds in general. We propose a sample of 20 binary targets, capitalizing on this unique combination of illumination by companion starlight, and collision with a companion wind, to probe wind attributes over a range in wind strengths. Of particular interest is the hypothesis that the radial distribution of the wind acceleration is altered significantly, when the radiative transfer within the winds becomes optically thick to resonance scattering in multiple overlapping UV lines.
PDF