325 research outputs found
Asymptotic Giant Branch Variables in the Galaxy and the Local Group
AGB variables, particularly the large amplitude Mira type, are a vital step
on the distance scale ladder. They will prove particularly important in the era
of space telescopes and extremely large ground-based telescopes with adaptive
optics, which will be optimized for infrared observing. Our current
understanding of the distances to these stars is reviewed with particular
emphasis on improvements that came from Hipparcos as well as on recent work on
Local Group galaxies. In addition to providing the essential calibration for
extragalactic distances Gaia may also provide unprecedented insight into the
poorly understood mass-loss process itself.Comment: Accepted for publication in Astrophysics and Space Science. From a
presentation at the conference "The Fundamental Cosmic Distance Scale: State
of the Art and Gaia Perspective, Naples May 2011. 8 Pages, 9 Figure
The Galactic Kinematics of Mira Variables
The galactic kinematics of Mira variables derived from radial velocities,
Hipparcos proper motions and an infrared period-luminosity relation are
reviewed. Local Miras in the 145-200day period range show a large asymmetric
drift and a high net outward motion in the Galaxy. Interpretations of this
phenomenon are considered and (following Feast and Whitelock 2000) it is
suggested that they are outlying members of the bulge-bar population and
indicate that this bar extends beyond the solar circle.Comment: 7 pages, 2 figure, to be published in Mass-Losing Pulsating Stars and
their Circumstellar Matter, Y. Nakada & M. Honma (eds) Kluwer ASSL serie
New observations of cool carbon stars in the halo
We report new results of our search for rare, cool carbon stars located at
large distances from the galactic plane. Eighteen new C stars were discovered.
Six are remarkable by showing the two peculiarities of a strong infrared excess
at 12 microns and a large height above the Galactic plane, from 1.7 to 6 kpc.
The number of C stars with these properties has been increased to 16. Mass-loss
rates were tentatively estimated by assuming that all these 16 stars are Miras
and by using the correlation between Mdot and the K-12 colour index. It is
found that several stars have large mass loss, with median Mdot around 4E-06
solar mass per year. It would be desirable to detect their CO emission ...
Eight stars might be at more than 30 kpc from the Sun, and two at the
unprecedented distance of 150 kpc (this abstract has been abridged).Comment: 15 pages; accepted by Astronomy and Astrophysic
The Stellar Population of the M31 Spiral Arm Around OB Association A24
A study of the stellar population of the M31 spiral arm around OB association
A24 was carried out based on the photometric data obtained from deep V and JHK
imaging. The luminosity function was obtained for -7 <~ Mbol <~ -3.5 by
applying the extinction correction corresponding to Av=1 and the bolometric
correction BC(K) as an empirical function of (J-K)o. In comparing the observed
color-luminosity diagrams with semitheoretical isochrones modified for the
dust-shell effects, we found the young population of t <~ 30 Myr with
supergiants of Mbol <~ -5, the bulk of the intermediate-age population of t ~
0.2 - 2.5 Gyr with bright asymptotic giant branch (AGB) stars of -5 <~ Mbol <~
-4, and old populations of t ~> 3 Gyr with AGB and red giant branch (RGB) stars
of Mbol ~> -4. The average star formation rate was estimated to be ~1.8x10^4
M_o/Myr and ~0.7x10^4 M_o/Myr per deprojected disk area of 1 kpc^2 from the
number density of B0 V stars around Mv=-4.0 (age ~10 Myr) and the number
density of bright AGB stars around Mbol = -4.3 (age ~1 Gyr), respectively. A
study of the local variation in the V and the J and H luminosity functions
revealed a kind of anticorrelation between the population of the young
component and that of the intermediate-age component when subdomains of ~100 pc
scales were concerned. This finding suggests that the disk domain around the
A24 area experienced a series of star formation episodes alternatively among
different subdomains with a timescale of a few spiral passage periods. Brief
discussions are given about the interstellar extinction and about the lifetimes
of bright AGB stars and the highly red objects (HROs) in the same area.Comment: 27 pages, 11 figures, accepted: ApJ, July 1, 199
Axial Symmetry and Rotation in the SiO Maser Shell of IK Tauri
We observed v=1, J=1-0 43-GHz SiO maser emission toward the Mira variable IK
Tauri (IK Tau) using the Very Long Baseline Array (VLBA). The images resulting
from these observations show that SiO masers form a highly elliptical ring of
emission approximately 58 x 32 mas with an axial ratio of 1.8:1. The major axis
of this elliptical distribution is oriented at position angle of ~59 deg. The
line-of-sight velocity structure of the SiO masers has an apparent axis of
symmetry consistent with the elongation axis of the maser distribution.
Relative to the assumed stellar velocity of 35 km/s, the blue- and red-shifted
masers were found to lie to the northwest and southeast of this symmetry axis
respectively. This velocity structure suggests a NW-SE rotation of the SiO
maser shell with an equatorial velocity, which we determine to be ~3.6 km/s.
Such a NW-SE rotation is in agreement with a circumstellar envelope geometry
invoked to explain previous water and OH maser observations. In this geometry,
water and OH masers are preferentially created in a region of enhanced density
along the NE-SW equator orthogonal to the rotation/polar axis suggested by the
SiO maser velocities.Comment: 17 Pages, 4 figures (2 color); accepted for publication in Ap
Using student experience to inform the design of an automated feedback system for essay answers
The SAFeSEA project (Supportive Automated Feedback for Short Essay Answers) aims to develop an automated feedback system to support university students as they write summative essays. Empirical studies carried out in the initial phase of the system’s development illuminated students’ approaches to and understandings of the essay-writing process. Findings from these studies suggested that, regardless of their experience of higher education, students consider essay writing as: 1) a sequential set of activities, 2) a process that is enhanced through particular sources of support and 3) a skill that requires the development of personal strategies. Further data collected from tutors offered insight into the feedback and reflection stages of essay writing. These perspectives offer important considerations for the ongoing, iterative development of this automated feedback system and indeed, for any institution developing tools to support students’ writing
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Innovating Pedagogy 2015: Open University Innovation Report 4
This series of reports explores new forms of teaching, learning and assessment for an interactive world, to guide teachers and policy makers in productive innovation. This fourth report proposes ten innovations that are already in currency but have not yet had a profound influence on education. To produce it, a group of academics at the Institute of Educational Technology in The Open University collaborated with researchers from the Center for Technology in Learning at SRI International. We proposed a long list of new educational terms, theories, and practices. We then pared these down to ten that have the potential to provoke major shifts in educational practice, particularly in post-school education. Lastly, we drew on published and unpublished writings to compile the ten sketches of new pedagogies that might transform education. These are summarised below in an approximate order of immediacy and timescale to widespread implementation
The Central Regions of M31 in the 3 - 5 micron Wavelength Region
Images obtained with NIRI on the Gemini North telescope are used to
investigate the photometric properties of the central regions of M31 in the 3 -
5 micron wavelength range. The light distribution in the central arcsecond
differs from what is seen in the near-infrared in the sense that the difference
in peak brigh tness between P1 and P2 is larger in M' than in K'; no obvious
signature of P3 is dete cted in M'. These results can be explained if there is
a source of emission that contributes ~ 20% of the peak M' light of P1 and has
an effective temperature of no more than a few hundred K that is located
between P1 and P2. Based on the red K-M' color of this source, it is suggested
that the emission originates in a circumstellar dust shell surrounding a single
bright AGB star. A similar bright source that is ~ 8 arcsec from the center of
the galaxy is also detected in M'. Finally, the (L', K-L') color-magnitude
diagram of unblended stars shows a domin ant AGB population with photometric
characteristics that are similar to those of the most luminous M giants in the
Galactic bulge.Comment: To appear in the Astronomical Journa
Long Period Variable Stars: galactic populations and infrared luminosity calibrations
In this paper HIPPARCOS astrometric and kinematic data are used to calibrate
both infrared luminosities and kinematical parameters of Long Period Variable
stars (LPVs). Individual absolute K and IRAS 12 and 25 luminosities of 800 LPVs
are determined and made available in electronic form.
The estimated mean kinematics is analyzed in terms of galactic populations.
LPVs are found to belong to galactic populations ranging from the thin disk to
the extended disk. An age range and a lower limit of the initial mass is given
for stars of each population. A difference of 1.3mag in K for the upper limit
of the Asymptotic Giant Branch is found between the disk and old disk galactic
populations, confirming its dependence on the mass in the main sequence.
LPVs with a thin envelope are distinguished using the estimated mean IRAS
luminosities. The level of attraction (in the classification sense) of each
group for the usual classifying parameters of LPVs (variability and spectral
types) is examined.Comment: Accepted for publication in A&A (May 2001). 13 page
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