29 research outputs found
Flash Mixing of the White Dwarf Cooling Curve Spectroscopic Confirmation in NGC 2808
We present new HST far-UV spectroscopy of two dozen hot evolved stars in NGC 2808, a massive globular cluster with a large population of "blue-hook" stars. The blue-hook stars are found in ultraviolet color-magnitude diagrams of the most massive globular clusters, where they fall at luminosities immediately below the hot end of the horizontal branch (HB), in a region of the HR diagram unexplained by canonical stellar evolution theory. Using new theoretical evolutionary and atmospheric models, we have shown that these subluminous HB stars are very likely the progeny of stars that undergo extensive internal mixing during a late He-core flash on the white dwarf cooling curve. This flash mixing leads to hotter temperatures and an enormous enhancement of the surface He and C abundances; the hotter temperatures and associated decrease in the hydrogen opacity shortward of the Lyman limit makes the stars brighter in the extreme UV but appear sub luminous in the UV and optical. Our far-UV spectroscopy demonstrates that, relative to normal HB stars at the same color, the blue-hook stars of NGC 2808 are hotter and greatly enhanced in He and C, thus providing unambiguous evidence of flash mixing in the subluminous population. Although the C abundance in the blue-hook stars is orders of magnitude larger than that in the normal HB stars, the atmospheric C abundance in both the blue-hook and normal HB stars appears to be affected by gravitational settling. The abundance variations seen in C, Si, and the Fe-peak elements indicate that atmospheric diffusion is at play in our sample, with all of our hot subdwarfs at 25,000 K to 50,000 K exhibiting large enhancements of the iron-peak elements. The hottest subdwarfs in our blue-hook sample may be pulsators, given that they fall in the temperature range of newly-discovered pulsating subdwarfs in omega Cen
New Observational Evidence of Flash Mixing on the White Dwarf Cooling Curve
Blue hook stars are a class of subluminous extreme horizontal branch stars
that were discovered in UV images of the massive globular clusters omega Cen
and NGC 2808. These stars occupy a region of the HR diagram that is unexplained
by canonical stellar evolution theory. Using new theoretical evolutionary and
atmospheric models, we have shown that the blue hook stars are very likely the
progeny of stars that undergo extensive internal mixing during a late
helium-core flash on the white dwarf cooling curve. This "flash mixing"
produces hotter-than-normal EHB stars with atmospheres significantly enhanced
in helium and carbon. The larger bolometric correction, combined with the
decrease in hydrogen opacity, makes these stars appear subluminous in the
optical and UV. Flash mixing is more likely to occur in stars born with a high
helium abundance, due to their lower mass at the main sequence turnoff. For
this reason, the phenomenon is more common in those massive globular clusters
that show evidence for secondary populations enhanced in helium. However, a
high helium abundance does not, by itself, explain the presence of blue hook
stars in massive globular clusters. Here, we present new observational evidence
for flash mixing, using recent HST observations. These include UV
color-magnitude diagrams of six massive globular clusters and far-UV
spectroscopy of hot subdwarfs in one of these clusters (NGC 2808).Comment: To appear in the Proceedings of the Fifth Meeting on Hot Subdwarf
Stars and Related Objects (sdOB5), 25 - 29 July 2011, Stellenbosch, South
Africa. ASP Conference Series, 10 pages, 5 figures, LaTe
The Mid-Infrared Instrument for the James Webb Space Telescope, VIII: The MIRI Focal Plane System
We describe the layout and unique features of the focal plane system for
MIRI. We begin with the detector array and its readout integrated circuit
(combining the amplifier unit cells and the multiplexer), the electronics, and
the steps by which the data collection is controlled and the output signals are
digitized and delivered to the JWST spacecraft electronics system. We then
discuss the operation of this MIRI data system, including detector readout
patterns, operation of subarrays, and data formats. Finally, we summarize the
performance of the system, including remaining anomalies that need to be
corrected in the data pipeline
vbyCaHbeta CCD Photometry of Clusters. VI. The Metal-Deficient Open Cluster NGC 2420
CCD photometry on the intermediate-band vbyCaHbeta system is presented for
the metal-deficient open cluster, NGC 2420. Restricting the data to probable
single members of the cluster using the CMD and the photometric indices alone
generates a sample of 106 stars at the cluster turnoff. The average E(b-y) =
0.03 +/- 0.003 (s.e.m.) or E(B-V) = 0.050 +/- 0.004 (s.e.m.), where the errors
refer to internal errors alone. With this reddening, [Fe/H] is derived from
both m1 and hk, using b-y and Hbeta as the temperature index. The agreement
among the four approaches is reasonable, leading to a final weighted average of
[Fe/H] = -0.37 +/- 0.05 (s.e.m.) for the cluster, on a scale where the Hyades
has [Fe/H] = +0.12. When combined with the abundances from DDO photometry and
from recalibrated low-resolution spectroscopy, the mean metallicity becomes
[Fe/H] = -0.32 +/- 0.03. It is also demonstrated that the average cluster
abundances based upon either DDO data or low-resolution spectroscopy are
consistently reliable to 0.05 dex or better, contrary to published attempts to
establish an open cluster metallicity scale using simplistic offset corrections
among different surveys.Comment: scheduled for Jan. 2006 AJ; 33 pages, latex, includes 7 figures and 2
table
Flash Mixing on the White Dwarf Cooling Curve: Spectroscopic Confirmation in NGC 2808
[Abridged] We present new HST FUV spectroscopy of 24 hot evolved stars in
NGC2808, a massive globular cluster with a large population of "blue-hook"
(BHk) stars. The BHk stars are found in UV color-magnitude diagrams of the most
massive globular clusters, where they fall at luminosities immediately below
the hot end of the horizontal branch (HB), in a region of the HR diagram
unexplained by canonical stellar evolution theory. Using new evolutionary and
atmospheric models, we have shown that these subluminous HB stars are very
likely the progeny of stars that undergo extensive internal mixing during a
late He-core flash on the white dwarf cooling curve. This flash mixing leads to
hotter temperatures and an enormous enhancement of the surface He and C; these
hotter temperatures, together with the decrease in H opacity shortward of the
Lyman limit, make the BHk stars brighter in the EUV while appearing subluminous
in the UV and optical. Our FUV spectroscopy demonstrates that, relative to
normal HB stars at the same color, the BHk stars of NGC2808 are hotter and
greatly enhanced in He and C, thus providing unambiguous evidence of flash
mixing. Although the C abundance in the BHk stars is orders of magnitude larger
than that in the normal HB stars, the C abundance in both the BHk and normal HB
stars appears to be affected by gravitational settling. The variations seen in
Si and the Fe-peak elements also indicate that atmospheric diffusion is at play
in our sample, with all of our hot subdwarfs at 25,000 to 50,000 K exhibiting
large enhancements of the Fe-peak elements. The hottest subdwarfs in our BHk
sample may be pulsators, given that they fall in the temperature range of
newly-discovered pulsating subdwarfs in omega Cen. In addition to the normal
hot HB and BHk stars, we obtain spectra of 5 blue HB stars, a post-HB star, and
3 unclassified stars with unusually blue UV colors.Comment: Accepted for publication in The Astrophysical Journal. LaTeX, 17
pages, 14 color figure
Mapping Circumstellar Matter with Polarized Light: The Case of Supernova 2014J in M82
Optical polarimetry is an effective way of probing the environment of a supernova for dust. We acquired linear
HST ACS/WFC polarimetry in bands F W 475 , F606W, and F775W of the supernova (SN) 2014J in M82 at six
epochs from ∼277 days to ∼1181 days after the B-band maximum. The polarization measured at day 277 shows
conspicuous deviations from other epochs. These differences can be attributed to at least ∼10-6 M of
circumstellar dust located at a distance of ~ ´5 10 c 17 m from the SN. The scattering dust grains revealed by these
observations seem to be aligned with the dust in the interstellar medium that is responsible for the large reddening
toward the supernova. The presence of this circumstellar dust sets strong constraints on the progenitor system that
led to the explosion of SN 2014J; however, it cannot discriminate between single- and double-degenerate models
V838 Monocerotis: A Geometric Distance from Hubble Space Telescope Polarimetric Imaging of its Light Echo
Following the outburst of the unusual variable star V838 Monocerotis in 2002,
a spectacular light echo appeared. A light echo provides the possibility of
direct geometric distance determination, because it should contain a ring of
highly linearly polarized light at a linear radius of ct, where t is the time
since the outburst. We present imaging polarimetry of the V838 Mon light echo,
obtained in 2002 and 2005 with the Advanced Camera for Surveys onboard the
Hubble Space Telescope, which confirms the presence of the highly polarized
ring. Based on detailed modeling that takes into account the outburst light
curve, the paraboloidal echo geometry, and the physics of dust scattering and
polarization, we find a distance of 6.1+-0.6 kpc. The error is dominated by the
systematic uncertainty in the scattering angle of maximum linear polarization,
taken to be theta_{max}=90^o +- 5^o. The polarimetric distance agrees
remarkably well with a distance of 6.2+-1.5 kpc obtained from the entirely
independent method of main-sequence fitting to a sparse star cluster associated
with V838 Mon. At this distance, V838 Mon at maximum light had M_V\simeq-9.8,
making it temporarily one of the most luminous stars in the Local Group. Our
validation of the polarimetric method offers promise for measurement of
extragalactic distances using supernova light echoes.Comment: 43 pages, 17 figures, 3 tables; accepted for publication in the
Astronomical Journal. Version with high-quality figures available at
http://www.stsci.edu/~bond/v838monpolariz.pd
Interstellar-medium mapping in M82 through light echoes around supernova 2014J
We present multiple-epoch measurements of the size and surface brightness of the light echoes from supernova (SN) 2014J in the nearby starburst galaxy M82. Hubble Space Telescope (HST) ACS/WFC images were taken ~277 and ~416 days after B-band maximum in the filters F475W, F606W, and F775W. Observations with HST WFC3/UVIS images at epochs ~216 and ~365 days are included for a more complete analysis. The images reveal the temporal evolution of at least two major light-echo components. The first one exhibits a filled ring structure with position-angle-dependent intensity. This radially extended, diffuse echo indicates the presence of an inhomogeneous interstellar dust cloud ranging from ~100 to ~500 pc in the foreground of the SN. The second echo component appears as an unresolved luminous quarter-circle arc centered on the SN. The wavelength dependence of scattering measured in different dust components suggests that the dust producing the luminous arc favors smaller grain sizes, while that causing the diffuse light echo may have sizes similar to those of the Milky Way dust. Smaller grains can produce an optical depth consistent with that along the supernova-Earth line of sight measured by previous studies around maximum light. Therefore, it is possible that the dust slab from which the luminous arc arises is also responsible for most of the extinction toward SN 2014J. The optical depths determined from the Milky Way-like dust in the scattering matters are lower than the optical depth produced by the dust slab
The Mid-infrared Instrument for JWST and Its In-flight Performance
The Mid-Infrared Instrument (MIRI) extends the reach of the James Webb Space Telescope (JWST) to 28.5 μm. It provides subarcsecond-resolution imaging, high sensitivity coronagraphy, and spectroscopy at resolutions of λ/Δλ ∼ 100-3500, with the high-resolution mode employing an integral field unit to provide spatial data cubes. The resulting broad suite of capabilities will enable huge advances in studies over this wavelength range. This overview describes the history of acquiring this capability for JWST. It discusses the basic attributes of the instrument optics, the detector arrays, and the cryocooler that keeps everything at approximately 7 K. It gives a short description of the data pipeline and of the instrument performance demonstrated during JWST commissioning. The bottom line is that the telescope and MIRI are both operating to the standards set by pre-launch predictions, and all of the MIRI capabilities are operating at, or even a bit better than, the level that had been expected. The paper is also designed to act as a roadmap to more detailed papers on different aspects of MIRI