arXiv subjects

Linda E. Strubbe

Publications and source records attributed to Linda E. Strubbe.

12 recordsLinked to original sources

arXiv2020

PhysPort use and growth: Supporting physics teaching with research-based resources since 2011

PhysPort (www.physport.org) has become the go-to place for physics faculty to learn to apply research-based teaching and assessment in their classrooms. Usage has doubled every two years since the site was released in 2011, and 20% of all U.S. physics faculty are now verified educators on PhysPort. The lead author conceived of PhysPort in 2007 after meeting many physics faculty interested in incorporating results of physics education research (PER) in their classrooms but with no idea where to start. At the time, most PER results were buried in research journals not accessible to ordinary physics educators, and there was no central place to learn about results and implications for classroom practice. Throughout its development, PhysPort has been based on user research: we interview physics faculty about their needs, design the site based on those needs, and conduct usability testing to see how we meet those needs. PhysPort is a joint product of the American Association of Physics Teachers (AAPT) and Kansas State University (K-State), with contributions from many universities, funded by the National Science Foundation (NSF), and a library within the ComPADRE.org digital library. The site (originally called the PER User's Guide) was first funded by an NSF grant in 2009 and released in November 2011. This article presents an overview of resources on PhysPort, discussion of research and development of the site, and data on the continuing growth of site usage.

physics.ed-ph↗

arXiv2020

Beyond teaching methods: highlighting physics faculty's strengths and agency

Much work in physics education research (PER) characterizes faculty teaching practice in terms of whether faculty use specific named PER-based teaching methods, either with fidelity or with adaptation; we call this research paradigm the "teaching-method-centered paradigm." However, most faculty do not frame their teaching in terms of which particular named methods they use, but rather in terms of their own ideas and values, suggesting that the teaching-method-centered paradigm misses key features of faculty teaching. These key features include the productive ideas that faculty have about student learning and faculty agency around teaching. We present three case studies of faculty talking about their teaching, and analyze them in terms of two theoretical frameworks: a framework of teaching principles (How Learning Works) and a framework of faculty agency (Self-Determination Theory). We show that these frameworks well characterize key features of faculty teaching practices and agency, and can be combined in a new paradigm for modeling faculty teaching which we call an "asset-based agentic paradigm." We therefore encourage physics education researchers to move beyond the teaching-method-centered paradigm and think about faculty teaching using an asset-based agentic paradigm.

physics.ed-ph↗

arXiv2019

Physics departments should discuss sexual harassment -- But consider this first

In April 2019, Aycock et al. published "Sexual harassment reported by undergraduate female physicists" in Phys. Rev. PER. Their main finding is that 3/4 of undergraduate women in physics in the U.S. report experiencing sexual harassment. Gender minorities experience high rates of harassment also. Many physics departments will want to discuss these findings with the ultimate goal of making our field harassment-free. However, there are major challenges inherent in having these discussions, since many participants will have experienced harassment themselves. We suggest questions for discussion organizers to reflect on, and ideas for how departments can approach these discussions.

physics.ed-ph↗

arXiv2016

Paired teaching for faculty professional development in teaching

Paired (or co-)teaching is an arrangement in which two faculty are collaboratively responsible for all aspects of teaching a course. By pairing an instructor experienced in research-based instructional strategies (RBIS) with an instructor with little or no experience in RBIS, paired teaching can be used to promote the adoption of RBIS. Using data from post-course interviews with the novice instructors of four such arrangements, we seek to describe factors that make for effective professional development in teaching via paired teaching. We suggest that the novice instructor's approach to the paired teaching and their previous teaching experience are two aspects which mediate their learning about teaching. Additionally, the structure of the pair-taught course and the sequence of teaching assignments for the novice both likely play roles in lowering the barrier to novice instructors adopting RBIS. We discuss these results within the framework of cognitive apprenticeship.

physics.ed-ph↗

arXiv2016

An Ultraviolet Spectrum of the Tidal Disruption Flare ASASSN-14li

We present a Hubble Space Telescope STIS spectrum of ASASSN-14li, the first rest-frame UV spectrum of a tidal disruption flare (TDF). The underlying continuum is well fit by a blackbody with $T_{\mathrm{UV}} = 3.5 \times 10^{4}$ K, an order of magnitude smaller than the temperature inferred from X-ray spectra (and significantly more precise than previous efforts based on optical and near-UV photometry). Super-imposed on this blue continuum, we detect three classes of features: narrow absorption from the Milky Way (probably a high-velocity cloud), and narrow absorption and broad (FWHM $\approx 2000$-8000 km s$^{-1}$) emission lines at/near the systemic host velocity. The absorption lines are blueshifted with respect to the emission lines by $Δv = -(250$-400) km s$^{-1}$. Due both to this velocity offset and the lack of common low-ionization features (Mg II, Fe II), we argue these arise from the same absorbing material responsible for the low-velocity outflow discovered at X-ray wavelengths. The broad nuclear emission lines display a remarkable abundance pattern: N III], N IV], He II are quite prominent, while the common quasar emission lines of C III] and Mg II are weak or entirely absent. Detailed modeling of this spectrum will help elucidate fundamental questions regarding the nature of the emission process(es) at work in TDFs, while future UV spectroscopy of ASASSN-14li would help to confirm (or refute) the previously proposed connection between TDFs and "N-rich" quasars.

astro-ph.HE↗

arXiv2015

Insights into Tidal Disruption of Stars from PS1-10jh

Was PS1-10jh (Gezari et al. 2012), an optical/UV transient discovered by the Pan-STARRS Medium Deep Survey, the tidal disruption of a star by a massive black hole? We address two aspects of the problem: the composition of the putative disrupted object (using the spectroscopic data), and the energetics of the observed gas and radiation (using the photometric data). We perform photoionization calculations and compare with the observed lower limit of the line ratio L(He II 4686)/L(Halpha) > 5 to argue that this event was not the disruption of a solar-type star, and instead was likely the disruption of a helium core (as first proposed by Gezari et al. 2012). Disruption of such a dense object requires a relatively small central BH, M_BH <~ 2 x 10^5 M_sun. We use the photometric data to infer that PS1-10jh comprised an outflow of ~ 0.01 M_sun of gas, escaping from the BH at ~1000 km/s, and we propose that this outflow was driven primarily by radiation pressure trapped by Thomson and resonance line scattering. The large ratio of radiated energy to kinetic energy, E_rad/E_K ~ 10^4, together with the large value of E_rad ~ 2 x 10^(51) erg, suggests that the outflow was shocked at large radius (perhaps similar to super-luminous supernovae or the internal shock model for gamma-ray bursts). We describe puzzles in the physics of PS1-10jh, and discuss how this event may help us understand future tidal disruptions and super-Eddington accretion events as well.

astro-ph.HE↗

arXiv2012

A College-Level Inquiry-Based Laboratory Activity on Transiting Planets

We have designed an inquiry-based laboratory activity on transiting extrasolar planets for an introductory college-level astronomy class. The activity was designed with the intent of simultaneously teaching science process skills and factual content about transits and light curves. In the activity, groups of two to four students each formulate a specific science question and design and carry out an investigation using a table-top model of a star and orbiting planet. Each group then presents their findings to other students in their class. In a final presentation, the instructors integrate students' findings with a summary of how measured light curves indicate properties of planetary systems. The activity debuted at Hartnell College in November 2009 and has also been adapted for a lecture-based astronomy course at U.C. Santa Cruz. We present the results of student surveys before and after the astronomy course at Hartnell and discuss how well our activity promotes students' confidence and identity as scientists, relative to traditional lab activities.

astro-ph.EP↗

arXiv2011

PTF10iya: A short-lived, luminous flare from the nuclear region of a star-forming galaxy

We present the discovery and characterisation of PTF10iya, a short-lived (dt ~ 10 d, with an optical decay rate of ~ 0.3 mag per d), luminous (M_g ~ -21 mag) transient source found by the Palomar Transient Factory. The ultraviolet/optical spectral energy distribution is reasonably well fit by a blackbody with T ~ 1-2 x 10^4 K and peak bolometric luminosity L_BB ~ 1-5 x 10^44 erg per s (depending on the details of the extinction correction). A comparable amount of energy is radiated in the X-ray band that appears to result from a distinct physical process. The location of PTF10iya is consistent with the nucleus of a star-forming galaxy (z = 0.22405 +/- 0.00006) to within 350 mas (99.7 per cent confidence radius), or a projected distance of less than 1.2 kpc. At first glance, these properties appear reminiscent of the characteristic "big blue bump" seen in the near-ultraviolet spectra of many active galactic nuclei (AGNs). However, emission-line diagnostics of the host galaxy, along with a historical light curve extending back to 2007, show no evidence for AGN-like activity. We therefore consider whether the tidal disruption of a star by an otherwise quiescent supermassive black hole may account for our observations. Though with limited temporal information, PTF10iya appears broadly consistent with the predictions for the early "super-Eddington" phase of a solar-type star disrupted by a ~ 10^7 M_sun black hole. Regardless of the precise physical origin of the accreting material, the large luminosity and short duration suggest that otherwise quiescent galaxies can transition extremely rapidly to radiate near the Eddington limit; many such outbursts may have been missed by previous surveys lacking sufficient cadence.

astro-ph.HE↗

arXiv2011

A relativistic jetted outburst from a massive black hole fed by a tidally disrupted star

While gas accretion onto some massive black holes (MBHs) at the centers of galaxies actively powers luminous emission, the vast majority of MBHs are considered dormant. Occasionally, a star passing too near a MBH is torn apart by gravitational forces, leading to a bright panchromatic tidal disruption flare (TDF). While the high-energy transient Swift J164449.3+573451 ("Sw 1644+57") initially displayed none of the theoretically anticipated (nor previously observed) TDF characteristics, we show that the observations (Levan et al. 2011) suggest a sudden accretion event onto a central MBH of mass ~10^6-10^7 solar masses. We find evidence for a mildly relativistic outflow, jet collimation, and a spectrum characterized by synchrotron and inverse Compton processes; this leads to a natural analogy of Sw 1644+57 with a smaller-scale blazar. The phenomenologically novel Sw 1644+57 thus connects the study of TDFs and active galaxies, opening a new vista on disk-jet interactions in BHs and magnetic field generation and transport in accretion systems.

astro-ph.HE↗

arXiv2011

Spectroscopic Signatures of the Tidal Disruption of Stars by Massive Black Holes

During the tidal disruption of a star by a massive black hole (BH) of mass MBH <~ 10^7 Msun, stellar debris falls back to the BH at a rate well above the Eddington rate. A fraction of this gas is subsequently blown away from the BH, producing an optically bright flare of radiation. We predict the spectra and spectral evolution of tidal disruption events, focusing on the photoionized gas outside this outflow's photosphere. The spectrum will show absorption lines that are strongly blueshifted relative to the host galaxy, very broad (0.01-0.1c), and strongest at UV wavelengths (e.g., C IV, Ly alpha, O VI), lasting ~ 1 month for a 10^6 Msun BH. Meanwhile, supernovae in galactic nuclei are a significant source of confusion in optical surveys for tidal disruption events: we estimate that nuclear Type Ia supernovae are two orders of magnitude more common than tidal disruption events at z ~ 0.1 for ground-based surveys. Nuclear Type II supernovae occur at a comparable rate but can be excluded by pre-selecting red galaxies. Supernova contamination can be reduced to a manageable level using high-resolution follow-up imaging with adaptive optics or the Hubble Space Telescope. Our predictions should help optical transient surveys capitalize on their potential for discovering tidal disruption events.

astro-ph.CO↗

arXiv2009

Optical Flares from the Tidal Disruption of Stars by Massive Black Holes

A star that wanders too close to a massive black hole (BH) is shredded by the BH's tidal gravity. Stellar gas falls back to the BH, releasing a flare of energy. In anticipation of upcoming transient surveys, we predict the light curves and spectra of tidal flares as a function of time, highlighting the unique signatures of tidal flares in the optical and near-IR. Some of the gas initially bound to the BH is likely blown away when the fallback rate is super-Eddington at early times. This outflow produces an optical luminosity comparable to that of a supernova; such events have durations of ~10 days and may have been missed in supernova searches that exclude the nuclear regions of galaxies. When the fallback rate subsides below Eddington, the gas accretes onto the BH via a thin disk whose emission peaks in the UV to soft X-rays. Some of this emission is reprocessed by the unbound stellar debris, producing a spectrum of very broad emission lines (with no corresponding narrow forbidden lines). These lines are strongest for BHs with MBH ~ 10^5 - 10^6 Msun and thus optical surveys are particularly sensitive to the lowest mass BHs in galactic nuclei. Calibrating our models to ROSAT and GALEX observations, we predict detection rates for Pan-STARRS, PTF, and LSST and highlight observational challenges in the optical. Pan-STARRS should detect at least several events per year--many more if current theoretical models of super-Eddington outflows are correct. These surveys will significantly improve our knowledge of stellar dynamics in galactic nuclei, the physics of super-Eddington accretion, the demography of intermediate mass BHs, and the role of tidal disruption in the growth of massive BHs.

astro-ph.CO↗

arXiv2006

Dust Dynamics, Surface Brightness Profiles, and Thermal Spectra of Debris Disks: The Case of AU Mic

AU Microscopii is a 12 Myr old M dwarf that harbors an optically thin, edge-on disk of dust. The scattered light surface brightness falls with projected distance b from the star as b^-a; within b = 43 AU, a = 1-2, while outside 43 AU, a = 4-5. We devise a theory to explain this profile. At a stellocentric distance r = r_BR = 43 AU, we posit a ring of parent bodies on circular orbits: the "birth ring," wherein micron-sized grains are born from the collisional attrition of parent bodies. The "inner disk" at r < r_BR contains grains that migrate inward by corpuscular and Poynting-Robertson (CPR) drag. The "outer disk" at r > r_BR comprises grains just large enough to remain bound to the star, on orbits rendered highly eccentric by stellar wind and radiation pressure. How the vertical optical depth tau scales with r depends on the fraction of grains that migrate inward by CPR drag without suffering a collision. If this fraction is large, the inner disk and birth ring share the same optical depth, and tau scales as r^-5/2 in the outer disk. By contrast, under collision-dominated conditions, the inner disk is empty, and tau scales as r^-3/2 outside. These scaling relations, which we derive analytically and confirm numerically, are robust against uncertainties in the grain size distribution. By simultaneously modeling the surface brightness and thermal spectrum, we break model degeneracies to establish that the AU Mic system is collision-dominated, and that its narrow birth ring contains a lunar mass of decimeter-sized bodies. The inner disk is devoid of micron-sized grains; the surface brightness at b < 43 AU arises from light forward scattered by the birth ring. Inside b = 43 AU, the disk's V-H color should not vary with b; outside, the disk must become bluer as ever smaller grains are probed.

astro-ph↗