arXiv subjects

Y. C. Unruh

Publications and source records attributed to Y. C. Unruh.

At least 37 records · Page 2Linked to original sources

arXiv2012

Reconstruction of total solar irradiance 1974-2009

Context: The study of variations in total solar irradiance (TSI) is important for understanding how the Sun affects the Earth's climate. Aims: Full-disk continuum images and magnetograms are now available for three full solar cycles. We investigate how modelled TSI compares with direct observations by building a consistent modelled TSI dataset. The model, based only on changes in the photospheric magnetic flux can then be tested on rotational, cyclical and secular timescales. Methods: We use Kitt Peak and SoHO/MDI continuum images and magnetograms in the SATIRE-S model to reconstruct TSI over cycles 21-23. To maximise independence from TSI composites, SORCE/TIM TSI data are used to fix the one free parameter of the model. We compare and combine the separate data sources for the model to estimate an uncertainty on the reconstruction and prevent any additional free parameters entering the model. Results: The reconstruction supports the PMOD composite as being the best historical record of TSI observations, although on timescales of the solar rotation the IRMB composite provides somewhat better agreement. Further to this, the model is able to account for 92% of TSI variations from 1978 to 2009 in the PMOD composite and over 96% during cycle 23. The reconstruction also displays an inter-cycle, secular decline of 0.20 (+0.12 / -0.09) Wm-2 between cycle 23 minima, in agreement with the PMOD composite. Conclusions: SATIRE-S is able to recreate TSI observations on all timescales of a day and longer over 31 years from 1978. This is strong evidence that changes in photospheric magnetic flux alone are responsible for almost all solar irradiance variations over the last three solar cycles.

astro-ph.SR↗

arXiv2010

Intensity contrast from MHD simulations and from HINODE observations

Changes in the solar surface area covered by small-scale magnetic elements are thought to cause long-term changes in the solar spectral irradiance, which are important for determining the impact on Earth's climate. To study the effect of small-scale magnetic elements on total and spectral irradiance, we derive their contrasts from 3-D MHD simulations of the solar atmosphere. Such calculations are necessary since measurements of small-scale flux tube contrasts are confined to a few wavelengths and suffer from scattered light and instrument defocus, even for space observations. To test the contrast calculations, we compare rms contrasts from simulations with those obtained with the broad-band filter imager mounted on the Solar Optical Telescope (SOT) onboard the Hinode satellite and also analyse centre-to-limb variations (CLV). The 3-D MHD simulations include the interaction between convection and magnetic flux tubes. They have been run with non-grey radiative transfer using the MURaM code. Simulations have an average vertical magnetic field of 0G, 50G, and 200G. Emergent intensities are calculated with the spectral synthesis code ATLAS9 and are convolved with a theoretical point-spread function to account for the properties of the observations' optical system. We find reasonable agreement for simulated and observed intensity distributions in the visible continuum bands. Agreement is poorer for the CN and G-Bands. The analysis of the simulations exhibits a potentially more realistic centre-to-limb behaviour than calculations based on 1-D model atmospheres. We conclude that starting from 3-D MHD simulations represents a powerful approach to obtaining intensity contrasts for a wide wavelength coverage and for different positions on the solar disk. This also paves the way for future calculations of facular and network contrast as a function of magnetic fluxes.

astro-ph.SR↗

arXiv2009

Dynamo Processes in the T Tauri star V410 Tau

We present new brightness and magnetic images of the weak-line T Tauri star V410 Tau, made using data from the NARVAL spectropolarimeter at Telescope Bernard Lyot (TBL). The brightness image shows a large polar spot and significant spot coverage at lower latitudes. The magnetic maps show a field that is predominantly dipolar and non-axisymmetric with a strong azimuthal component. The field is 50% poloidal and 50% toroidal, and there is very little differential rotation apparent from the magnetic images. A photometric monitoring campaign on this star has previously revealed V-band variability of up to 0.6 magnitudes but in 2009 the lightcurve is much flatter. The Doppler image presented here is consistent with this low variability. Calculating the flux predicted by the mapped spot distribution gives an peak-to-peak variability of 0.04 magnitudes. The reduction in the amplitude of the lightcurve, compared with previous observations, appears to be related to a change in the distribution of the spots, rather than the number or area. This paper is the first from a Zeeman-Doppler imaging campaign being carried out on V410 Tau between 2009-2012 at TBL. During this time it is expected that the lightcurve will return to a high amplitude state, allowing us to ascertain whether the photometric changes are accompanied by a change in the magnetic field topology.

astro-ph.SR↗

arXiv2009

Towards a long-term record of solar total and spectral irradiance

The variation of total solar irradiance (TSI) has been measured since 1978 and that of the spectral irradiance for an even shorter amount of time. Semi-empirical models are now available that reproduce over 80% of the measured irradiance variations. An extension of these models into the more distant past is needed in order to serve as input to climate simulations. Here we review our most recent efforts to model solar total and spectral irradiance on time scales from days to centuries and even longer. Solar spectral irradiance has been reconstructed since 1947. Reconstruction of solar total irradiance goes back to 1610 and suggests a value of about 1-1.5 Wm$^{-2}$ for the increase in the cycle-averaged TSI since the end of the Maunder minimum, which is significantly lower than previously assumed but agrees with other modern models. First steps have also been made towards reconstructions of solar total and spectral irradiance on time scales of millennia.

astro-ph.SR↗

arXiv2009

IPHAS and the symbiotic stars. II. New discoveries and a sample of the most common mimics

In a previous paper [arXiv:0712.2391], we presented the selection criteria needed to search for symbiotic stars in IPHAS, the INT Halpha survey of the Northern Galactic plane. IPHAS gives us the opportunity to make a systematic, complete search for symbiotic stars in a magnitude-limited volume. Follow-up spectroscopy at different telescopes worldwide of a sample of sixty two symbiotic star candidates is presented. Seven out of nineteen S-type candidates observed spectroscopically are confirmed to be genuine symbiotic stars. The spectral type of their red giant components, as well as reddening and distance, were computed by modelling the spectra. Only one new D-type symbiotic system, out of forty-three candidates observed, was found. This was as expected (see discussion in our paper on the selection criteria). The object shows evidence for a high density outflow expanding at a speed larger than 65 km/s. Most of the other candidates are lightly reddened classical T Tauri stars and more highly reddened young stellar objects that may be either more massive young stars of HAeBe type or classical Be stars. In addition, a few notable objects have been found, such as three new Wolf-Rayet stars and two relatively high-luminosity evolved massive stars. We also found a helium-rich source, possibly a dense ejecta hiding a WR star, which is surrounded by a large ionized nebula.

astro-ph.GA↗

arXiv2009

A comparison of measured and simulated solar network contrast

Long-term trends in the solar spectral irradiance are important to determine the impact on Earth's climate. These long-term changes are thought to be caused mainly by changes in the surface area covered by small-scale magnetic elements. The direct measurement of the contrast to determine the impact of these small-scale magnetic elements is, however, limited to a few wavelengths, and is, even for space instruments, affected by scattered light and instrument defocus. In this work we calculate emergent intensities from 3-D simulations of solar magneto-convection and validate the outcome by comparing with observations from Hinode/SOT. In this manner we aim to construct the contrast at wavelengths ranging from the NUV to the FIR.

astro-ph.SR↗

arXiv2009

The Structure of the Outer Galactic Disc as revealed by IPHAS early A Stars

This study is an investigation of the stellar density profile of the Galactic disc in the Anticentre direction. We select over 40,000 early A stars from IPHAS photometry in the Galactic longitude range 160 < l < 200 close to the equatorial plane (-1 < b < +1). We then compare their observed reddening-corrected apparent magnitude distribution with simulated photometry obtained from parameterised models in order to set constraints on the Anticentre stellar density profile. By selecting A stars, we are appraising the properties of a population only ~100 Myrs old. We find the stellar density profile of young stars is well fit to an exponential with length scale of (3020 \pm 120_{statistical} \pm 180_{systematic}) pc, which is comparable to that obtained in earlier studies, out to a Galactocentric radius of R_T = (13.0 \pm 0.5_{statistical} \pm 0.6_{systematic}) kpc. At larger radii the rate of decline appears to increase with the scale length dropping to (1200 \pm 300_{statistical} \pm 70_{systematic}) pc. This result amounts to a refinement of the conclusions reached in previous studies that the stellar density profile is abruptly truncated. The IPHAS A star data are not compatible with models that propose a sudden change in metallicity at R_G = 10 kpc.

astro-ph.GA↗

arXiv2009

Near-IR spectra of IPHAS extremely red Galactic AGB stars

We present a library of 139 near-IR spectra of cool asymptotic giant branch stars that will be useful for comparison with theoretical model atmosphere calculations and for modeling the integrated emission from intermediate-age stellar populations. The source list was selected from the `extremely red' region of the INT Photometric H Alpha Survey (IPHAS) colour-colour plane that is overwhelmingly dominated by very late-type stars. The spectral library also includes a large fraction of S-type and carbon stars. We present a number of spectral classification sequences highlighting the various molecular features identified and discuss a number of rare features with uncertain identifications in the literature. With its focus on particularly cool photospheres this catalogue serves as a companion to recent spectroscopic atlases of MK standards in the near-IR. Finally the relationship between IPHAS (r'-i') and (r'-H Alpha) colours and spectroscopically determined properties is discussed and a strong correlation between (r'-H Alpha) colour and the C/O abundance index for S-type and carbon stars is noted. This relation has the potential to separate O-rich, S-type and carbon stars in the Galaxy based on their photometry alone.

astro-ph.SR↗

arXiv2009

Doppler Images and Chromospheric Variability of TWA 17

We present Doppler imaging and a Balmer line analysis of the weak-line T Tauri star TWA 17. Spectra were taken in 2006 with the UCL Echelle Spectrograph on the Anglo-Australian Telescope. Using least-squares deconvolution to improve the effective signal-to-noise ratio we produced a Doppler map of the surface spot distribution. This shows similar features to maps of other rapidly rotating T Tauri stars, i.e. a polar spot with more spots extending out of it down to the equator. In addition to the photospheric variability, the chromospheric variability was studied using the Balmer emission. The mean H-alpha profile has a narrow component consistent with rotational broadening and a broad component extending out to +/-220 km/s. The variability in H-alpha suggests that the chromosphere has at least one slingshot prominence 3 stellar radii above the surface.

astro-ph.SR↗

arXiv2008

Extremely red stellar objects revealed by IPHAS

We present photometric analysis and follow-up spectroscopy for a population of extremely red stellar objects extracted from the point-source catalogue of the INT Photometric H-Alpha Survey (IPHAS) of the northern galactic plane. The vast majority of these objects have no previous identification. Analysis of optical, near- and mid-infrared photometry reveals that they are mostly highly-reddened asymptotic giant branch stars, with significant levels of circumstellar material. We show that the distribution of these objects traces galactic extinction, their highly reddened colours being a product of both interstellar and circumstellar reddening. This is the first time that such a large sample of evolved low-mass stars has been detected in the visual and allows optical counterparts to be associated with sources from recent infrared surveys. Follow-up spectroscopy on some of the most interesting objects in the sample has found significant numbers of S-type stars which can be clearly separated from oxygen-rich objects in the IPHAS colour-colour diagram. We show that this is due to the positions of different molecular bands relative to the narrow-band H-alpha filter used for IPHAS observations. The IPHAS (r' - H-alpha) colour offers a valuable diagnostic for identifying S-type stars. A selection method for identifying S-type stars in the galactic plane is briefly discussed and we estimate that over a thousand new objects of this type may be discovered, potentially doubling the number of known objects in this short but important evolutionary phase.

astro-ph↗

arXiv2008

An IPHAS-based search for accreting very low-mass objects using VO tools

The main goal of this paper is to prove that accreting very low-mass stars and brown dwarfs can be identified in IPHAS, a Halpha emission survey of the northern Milky Way.Full exploitation of the IPHAS database and a future extension of it in the southern hemisphere will be useful to identify very low-mass accreting objects near and far well-known star forming regions. We have used Virtual Observatory tools to cross-match the IPHAS catalogue with the 2MASS catalogue. We defined photometric criteria to identify Halpha emission sources with near-infrared colours similar to known young very low-mass stars and brown dwarfs. 4000 candidates were identified that met our criteria over an area of 1600 square degrees. We present low-resolution optical spectra of 113 candidates. Spectral types have been derived for the 33 candidates that have spectroscopically confirmed Halpha emission, negligible reddening and M spectral class. We have also measured Halpha emission and investigated the NaI doublet (818.3 nm, 819.5 nm) in these 33 objects. We confirm that 33 IPHAS candidates have strong Halpha indicative of disk accretion for their spectral type. 23 of them have spectral class M4 or later, of which 10 have classes in the range M5.5-M7.0 and thus could be very young brown dwarfs. Also many objects have weak NaI doublet, an indication of low surface gravity.We conclude that IPHAS provides a very valuable database to identify accreting very low-mass stars and brown dwarfs, and that Virtual Observatory tools provide an efficient method for identifying these objects over large areas of the sky. Based on our success rate of 23 Halpha emission objects with spectral type in the range M4-M7 out of 113 candidates with spectroscopic follow-up, we estimate that there could be hundreds of such objects in the full IPHAS survey.

astro-ph↗

arXiv2008

High Spatial Resolution Galactic 3D Extinction Mapping with IPHAS

We present an algorithm ({\scshape mead}, for `Mapping Extinction Against Distance') which will determine intrinsic ($r' - i'$) colour, extinction, and distance for early-A to K4 stars extracted from the IPHAS $r'/i'/\Halpha$ photometric database. These data can be binned up to map extinction in three dimensions across the northern Galactic Plane. The large size of the IPHAS database ($\sim 200$ million unique objects), the accuracy of the digital photometry it contains and its faint limiting magnitude ($r' \sim 20$) allow extinction to be mapped with fine angular ($ \sim 10 $ arcmin) and distance ($\sim 0.1$ ~kpc) resolution to distances of up to 10 kpc, outside the Solar Circle. High reddening within the Solar Circle on occasion brings this range down to $\sim 2$ kpc. The resolution achieved, both in angle and depth, greatly exceeds that of previous empirical 3D extinction maps, enabling the structure of the Galactic Plane to be studied in increased detail. {\scshape mead} accounts for the effect of the survey magnitude limits, photometric errors, unresolved ISM substructure, and binarity. The impact of metallicity variations, within the range typical of the Galactic disc is small. The accuracy and reliability of {\scshape mead} are tested through the use of simulated photometry created with Monte-Carlo sampling techniques. The success of this algorithm is demonstrated on a selection of fields and the results are compared to the literature.

astro-ph↗

arXiv2008

Initial Data Release from the INT Photometric H-alpha Survey of the Northern Galactic Plane (IPHAS)

The INT/WFC Photometric H-alpha Survey of the Northern Galactic Plane (IPHAS) is an imaging survey being carried out in H-alpha, r' and i' filters, with the Wide Field Camera (WFC) on the 2.5-metre Isaac Newton Telescope (INT) to a depth of r'=20 (10 sigma). The survey is aimed at revealing large scale structure in our local galaxy, and also the properties of key early and late populations making up the Milky Way. Mapping emission line objects enables a particular focus on objects in the young and old stages of stellar evolution ranging from early T-Tauri stars to late planetary nebulae. In this paper we present the IPHAS Initial Data Release, primarily a photometric catalogue of about 200 million unique objects, coupled with associated image data covering about 1600 square degrees in three passbands. We note how access to the primary data products has been implemented through use of standard virtual observatory publishing interfaces. Simple traditional web access is provided to the main IPHAS photometric catalogue, in addition to a number of common catalogues (such as 2MASS) which are of immediate relevance. Access through the AstroGrid VO Desktop opens up the full range of analysis options, and allows full integration with the wider range of data and services available through the Virtual Observatory. The IDR represents the largest dataset published primarily through VO interfaces to date, and so stands as an examplar of the future of survey data mining. Examples of data access are given, including a cross-matching of IPHAS photometry with sources in the UKIDSS Galactic Plane Survey that validates the existing calibration of the best data.

astro-ph↗

arXiv2008

Doppler Images and Chromospheric Variability of TWA 6

We present Doppler imaging and Balmer line analysis of the weak-line T Tauri star TWA 6. Using this data we have made one of the first attempts to measure differential rotation in a T Tauri star, and the first detection of a slingshot prominence in such a star. We also show the most direct evidence to date of the existence of solar-type plages in a star other than the Sun. Observations were made over six nights: 11-13th February 2006 and 18-20th February 2006, when spectra were taken with the UCL Echelle Spectrograph on the 3.9-m Anglo-Australian Telescope. Using least-squares deconvolution to improve the effective signal--to--noise ratio we produced two Doppler maps. These show similar features to maps of other rapidly rotating T Tauri stars, i.e. a polar spot with more spots extending out of it down to equator. Comparison of the two maps was carried out to measure the differential rotation. Cross-correlation and parameter fitting indicates that TWA 6 does not have detectable differential rotation. The Balmer emission of the star was studied. The mean H-alpha profile has a narrow component consistent with rotational broadening and a broad component extending out to 250km/s. The variability in H-alpha suggests that the chromosphere has active regions that are cospatial with the spots in the photosphere, similar to the 'plages' observed on the Sun. In addition the star has at least one slingshot prominence 3 stellar radii above the surface - the first such detection in a T Tauri star.

astro-ph↗

arXiv2005

Five WC9 stars discovered in the AAO/UKST H-alpha Survey

We report the discovery of 5 massive Wolf-Rayet (WR) stars resulting from a programme of follow-up spectroscopy of candidate emission line stars in the AAO/UKST Southern Galactic Plane H-alpha survey. The 6195-6775 angstrom spectra of the stars are presented and discussed. A WC9 class is assigned to all 5 stars through comparison of their spectra with those of known late-type WC stars, bringing the known total number of Galactic WC9 stars to 44. Whilst three of the five WC9 stars exhibit near infrared (NIR) excesses characteristic of hot dust emission -- as seen in the great majority of known WC9 stars -- we find that two of the stars show no discernible evidence of such excesses. This increases the number of known WC9 stars without NIR excesses to 7. Reddenings and distances for all 5 stars are estimated.

astro-ph↗

arXiv2004

Discovery of a WO star in the Scutum-Crux arm of the inner Galaxy

We report the discovery of only the fourth massive WO star to be found in the Milky Way, and only the seventh identified within the Local Group. This has resulted from the first observations made in a programme of follow-up spectroscopy of candidate emission line stars from the AAO/UK Schmidt Southern Galactic Plane H-alpha Survey. The optical spectrum of this star, to become WR 93b in the Catalogue of Galactic Wolf-Rayet stars, is presented and described. WR 93b is classified as WO3 and is shown to be highly reddened (E(B-V) = 2.1 pm 0.1). A recombination line analysis of the emission lines yields the abundance ratios C/He = 0.95 and O/He = 0.13 (by number). Comparisons at near infrared wavelengths of reddening corrected photometry between WR 93b and both of Sand 2 (WO3, D = 49 kpc) and Sand 5 (WO2, D = 1.75 kpc) yields a consistent distance to WR 93b of 3.4 kpc. Positioned at Galactic co-ordinates l = 353.27, b = -0.85, the star is most likely located in the Scutum-Crux Arm of the inner Milky Way. We note that none of the four Galactic WO stars lies significantly beyond the Solar Circle (with two well inside). Estimation of the wind terminal velocity in WR 93b at 5750 km/s makes this star the current wind speed record holder among all non-degenerate stars.

astro-ph↗

arXiv2004

Multi-Site observations of SU Aurigae

We present results from the 1996 MUSICOS (MUlti-SIte COntinuous Spectroscopy) campaign on the T Tauri star SU Aurigae. We find a 2.7-d periodicity in the He I (587.6 nm) line and somewhat longer, less well-pronounced periodicities in the Balmer lines and in Na D. Our observations support the suggestion that the wind and infall signatures are out of phase on SU Aur. We present Doppler images of SU Aur that have been obtained from least-squares deconvolved profiles. Images taken about one rotation apart show only limited overlap, in particular at low latitudes. This is in part due to limitations in signal-to-noise, and in part due to line profile deformations that arise from short-lived and/or non-surface features. The agreement at high latitudes is better and suggests that at least some longer-lived features are present. The analysis of Stokes V profiles yields a marginal magnetic field detection during one of the phases.

astro-ph↗

arXiv2003

Spot Sizes on Sun-like Stars

The total area coverage by starspots is of interest for a variety of reasons, but direct techniques only provide estimates of this important quantity. Sunspot areas exhibit a lognormal size distribution irrespective of the phase of the activity cycle, implying that most sunspots are small. Here we explore the consequences if starspot areas were similarly distributed. The solar data allow for an increase in the fraction of larger sunspots with increasing activity. Taking this difference between the size distribution at sunspot maximum and minimum, we extrapolate to higher activity levels, assuming different dependencies of the parameters of the lognormal distribution on total spot coverage. We find that even for very heavily spotted (hypothetical) stars a large fraction of the spots are smaller than the current resolution limit of Doppler images and might hence be missed on traditional Doppler maps.

astro-ph↗