The Soft X-ray Lightcurves of Partially Eclipsed Stellar Flares

Most stellar flares' soft X-ray lightcurves possess a `typical' morphology, which consists of a rapid rise followed by a slow exponential decay. However, a study of 216 of the brightest flares on 161 pre-main sequence stars, observed during the Chandra Orion-Ultradeep Project (COUP), showed that many flare lightcurves depart from this typical morphology. While this can be attributed to the superposition of multiple typical flares, we explore the possibility that the time-variable eclipsing of flares by their host stars may also be an important factor. We assume each flare is contained within a single, uniform plasma density magnetic loop and specify the intrinsic variation of the flare's emission measure with time. We consider rotational eclipse by the star itself, but also by circumstellar discs and flare-associated prominences. Based on this simple model, we generate a set of flares similar to those observed in the COUP database. Many eclipses simply reduce the flare's maximum emission measure or decay time. We conclude therefore that eclipses often pass undetected, but usually have only a modest influence on the flare emission measure profile and hence the derived loop lengths. We show that eclipsing can easily reproduce the observed atypical flare morphologies. The number of atypical modelled flare morphologies is however much less than that found in the COUP sample. The large number of observed atypical flare morphologies, therefore, must be attributed to other processes such as multiple flaring loops.Comment: 11 pages, 9 figure

Protoplanetary Disk Evolution around the Triggered Star Forming Region Cepheus B

The Cepheus B (CepB) molecular cloud and a portion of the nearby CepOB3b OB association, one of the most active regions of star formation within 1 kpc, have been observed with the IRAC detector on board the Spitzer Space Telescope. The goals are to study protoplanetary disk evolution and processes of sequential triggered star formation in the region. Out of ~400 pre-main sequence (PMS) stars selected with an earlier Chandra X-ray Observatory observation, 95% are identified with mid-infrared sources and most of these are classified as diskless or disk-bearing stars. The discovery of the additional >200 IR-excess low-mass members gives a combined Chandra+Spitzer PMS sample complete down to 0.5 Mo outside of the cloud, and somewhat above 1 Mo in the cloud. Analyses of the nearly disk-unbiased combined Chandra+Spitzer selected stellar sample give several results. Our major finding is a spatio-temporal gradient of young stars from the hot molecular core towards the primary ionizing O star HD 217086. This strongly supports the radiation driven implosion (RDI) model of triggered star formation in the region. The empirical estimate for the shock velocity of 1 km/s is very similar to theoretical models of RDI in shocked molecular clouds...ABRIDGED... Other results include: 1. agreement of the disk fractions, their mass dependency, and fractions of transition disks with other clusters; 2. confirmation of the youthfulness of the embedded CepB cluster; 3. confirmation of the effect of suppression of time-integrated X-ray emission in disk-bearing versus diskless systems.Comment: Accepted for publication in The Astrophysical Journal. 48 pages, 14 figures. For a version with high-quality figures, see http://www.astro.psu.edu/users/gkosta/RESEARCH/cepb_spitzer_chandra.pd

A Chandra Observation of the Obscured Star-Forming Complex W40

The young stellar cluster illuminating the W40 H II region, one of the nearest massive star forming regions, has been observed with the ACIS detector on board the Chandra X-ray Observatory. Due to its high obscuration, this is a poorly-studied stellar cluster with only a handful of bright stars visible in the optical band, including three OB stars identified as primary excitation sources. We detect 225 X-ray sources, of which 85% are confidently identified as young stellar members of the region. Two potential distances of the cluster, 260 pc and 600 pc, are used in the paper. Supposing the X-ray luminosity function to be universal, it supports a 600 pc distance as a lower limit for W40 and a total population of at least 600 stars down to 0.1 Mo under the assumption of a coeval population with a uniform obscuration. In fact, there is strong spatial variation in Ks-band-excess disk fraction and non-uniform obscuration due to a dust lane that is identified in absorption in optical, infrared and X-ray. The dust lane is likely part of a ring of material which includes the molecular core within W40. In contrast to the likely ongoing star formation in the dust lane, the molecular core is inactive. The star cluster has a spherical morphology, an isothermal sphere density profile, and mass segregation down to 1.5 Mo. However, other cluster properties, including a \leq{1} Myr age estimate and ongoing star formation, indicate that the cluster is not dynamically relaxed. X-ray diffuse emission and a powerful flare from a young stellar object are also reported.Comment: Accepted for publication in The Astrophysical Journal. 60 pages, 16 figure

Development of the technique for improving the structure of a magnetic field in the aperture of a quadrupole electromagnet with a superconducting winding

Представляє практичний iнтерес створення моделей магнiтного поля квадрупольного електромагнiту, якi дозволяють коригувати середньо-iнтегральнi коефiцiєнти магнiтної iндукцiї за допомогою змiни геометричних параметрiв конструкцiї магнiту. Мета роботи – розробка методу оптимiзацiї конструкцiї квадрупольного електромагнiту з надпровiдною обмоткою за критерiєм мiнiмуму величин неквадрупольних середньо-iнтегральних коефiцiєнтiв магнiтної iндукцiї в апертурi. Практичне застосування методу дозволяє оптимiзувати конструкцiю квадрупольного електромагнiту для мiнiмiзацiї середньо-iнтегральних по довжинi коефiцiєнтiв магнiтної iндукцiї на основi розрахунку геометричних параметрiв ярма i обмотки. Отриманi аналiтичнi вирази для розрахунку мiнiмiзованих середньо-iнтегральних по довжинi коефiцiєнтiв магнiтної iндукцiї, що створюються всерединi апертури квадрупольного електромагнiту заснованi на їх пропорцiйностi вкладам вiд струмової обмотки в залежностi вiд її положення вiдносно ярма. Встановленi експериментальним шляхом взаємозв'язки мiж середньо-iнтегральними по довжинi коефiцiєнтами магнiтної iндукцiї i параметрами конструкцiї покладенi в основу процедури практичного застосування способу полiпшення однорiдностi градiєнта магнiтної iндукцiї квадрупольного електромагнiту. Отриманi вирази дозволяють розрахувати необхiдну корекцiю геометричних параметрiв вже iснуючої конструкцiї для оптимiзацiї магнiтного поля всерединi апертури квадрупольних електромагнiтiв по заданим середньо-iнтегральним коефiцiєнтам. Приведенi результати оптимiзацiї конструкцiї магнiтоактивної частини триплету квадрупольних електромагнiтiв прискорювального комплексу проекту NICA. Оптимiзацiя проводилась на основi запропонованого способу, з мiнiмiзацiєю до рiвня 10‾⁵ неквадрупольних середньо-iнтегральних коефiцiєнтiв поперечних складових магнiтної iндукцiї, створюваної в апертурi електромагнiту з надпровiдною обмоткою