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Ayres, Lewis

doi:10.1017/cbo9780511780301.009

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“…To that end, all of the recent (post-SM4) deep (t exp ≥ 266 s) lamp exposures taken with E140M, or the various E140H settings, were de-archived and processed through calstis as science images (using the lamp exposure as its own reference wavecal to set the zero-point offset), then combined, again using the same ASTRAL coaddition protocols as applied to the WDs, to yield high-S/N H and M full-coverage FUV tracings. As anticipated from the previous experiments of this type (Ayres 2008(Ayres , 2010, there were no significant deviations of the lamp lines from their laboratory wavelengths over the FUV H and M intervals. The 1 σ dispersion of the measurements was 0.2 km s −1 for H (290 lines), at the discretization level for the reported wavelengths; and slightly larger, 0.35 km s −1 (310 lines) for M, but still equivalent to only 1/20-th of a resolution element.…”

Section: A Reference Wavelengthssupporting

confidence: 84%

“…A comparison of low-excitation line wavelengths, e.g., for O I, C I, S I, and Fe II, among the various atomic databases generally finds good agreement with the NIST Ritz values at the ±1 mÅ level, which corresponds to about 0.2 km s −1 at 1400Å. This is sufficient accuracy given the residual distortion errors in the STIS E140M wavelengths, perhaps several times that level (see Ayres 2008). Thus, the NIST Ritz values were adopted for this type of transition.…”

Section: A Reference Wavelengthsmentioning

confidence: 61%

“…The echelle orders in a given sub-exposure then were merged according to protocols developed for the Advanced Spectral Library Project (ASTRAL 2 ), to produce a coherent spectral tracing: flux densities, photometric errors, and data quality flags versus wavelength. One crucial feature of the post-pipeline step was to apply a correction for smallscale wavelength distortions identified in an earlier extensive study of the STIS wavelengths, based on deep exposures of the onboard calibration lamps (Ayres 2008(Ayres , 2010. A second key aspect involved an adjustment of the echelle blaze curves to minimize flux discontinuities over adjacent echelle orders.…”

Section: Reduction Of Stis Echellegramsmentioning

confidence: 99%

“…The first concern was minimized by considering the two different WDs (and their different accompanying photoionization regions), as well as the contrasting spectral resolutions (at least for G191B2B). The second concern was tested according to an approach outlined by Ayres (2008), whereby one runs deep exposures of the STIS wavecal lamps through the normal pipeline as if they were observations of an external target, and then post-processes them using the ASTRAL protocols, as for the other stars discussed here. Comparing the empirical positions of the lamp lines with the laboratory values then can reveal any significant deviations with wavelength, as a gauge of the coherency of the fully processed wavelength scales.…”

Section: A Reference Wavelengthsmentioning

confidence: 99%

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The Far-Ultraviolet Ups and Downs of Alpha Centauri

Ayres

2015

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Four years (2010)(2011)(2012)(2013)(2014) of semiannual pointings by Hubble Space Telescope Imaging Spectrograph (STIS) on nearby Alpha Centauri have yielded a detailed time history of far-ultraviolet (FUV: 1150-1700Å) emissions of the solar-like primary (A: G2 V) and the cooler, but more active, secondary (B: K1 V). This period saw A climbing out of a prolonged coronal X-ray minimum, as documented contemporaneously by Chandra, while B was rising to, then falling from, a peak of its long-term (∼ 8 yr) starspot cycle. The FUV fluxes of the primary were steady over most of the STIS period, although the [Fe XII] λ1242 coronal forbidden line (T ∼ 1.5 MK) partly mirrored the slowly rising X-ray fluxes. The FUV emissions of the secondary more closely tracked the rise and fall of its coronal luminosities, especially the "hot lines" like Si IV, C IV, and N V (T ∼ 0.8-2 × 10 5 K), and coronal [Fe XII] itself. The hot lines of both stars were systematically redshifted, relative to narrow chromospheric emissions, by several km s −1 , showing little change in amplitude over the 4-year period; especially for α Cen B, despite the significant evolution of its coronal activity. Further, the hot lines of both stars, individually and epoch-averaged, displayed non-Gaussian shapes, which most trivially could be decomposed into two components, one narrow (FWHM ∼ 25-45 km s −1 ), the other broad (60-80 km s −1 ). The bimodal Gaussian strategy had been applied previously to the α Cen stars, but this was the first opportunity to evaluate any time dependence. In fact, not much variation of the component properties was seen, even over the major cycle changes of B. Curiously, the line fluxes were about equally divided between the narrow and broad components for both stars. The fact that there is minimal activitydependence of the narrow/broad flux partition, as well as densities derived from O IV] line ratios, either during the cycle evolution of B, or between A and B, suggests that there is a dominant "quantum" of FUV surface activity that is relatively unchanged during the cycle, aside from the fractional area covered.The Chandra High-Resolution Camera (HRC) imaging of α Cen AB up to mid-2013 has been described by Ayres (2014), where details concerning the instrument configurations and data analysis can be found. Since that study, two additional Chandra pointings have been carried out, in late-2013 and mid-2014. These are summarized in Table 1. The new L X /L bol ratios, together with the previous time series, are illustrated in Figure 1.The solar values in Fig. 1 are 81 day averages (three rotations), over Cycles 23 and (beginning of) 24; error flags are 1 σ standard deviations of the daily measurements in each 81 day bin, a rough measure of temporal variability. A three-cycle average is shaded gray, projected into the future as dashed curves, highlighting the unusual extended minimum of Solar Cycle 23. Larger solid dots represent X-ray fluxes of α Cen AB from the Chandra HRC pointings (Y2000 and later) and four earlier epochs from the R...

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Section: A Reference Wavelengthssupporting

confidence: 84%

Section: A Reference Wavelengthsmentioning

confidence: 61%

Section: Reduction Of Stis Echellegramsmentioning

confidence: 99%

Section: A Reference Wavelengthsmentioning

confidence: 99%

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