Stefan Keller | The Australian National University (original) (raw)
Papers by Stefan Keller
We present the results from an analysis of Hubble Space Telescope High Resolution Camera data for... more We present the results from an analysis of Hubble Space Telescope High Resolution Camera data for the Large Magellanic Cloud microlensing event MACHO-LMC-5. By determining the parallax and proper motion of this object we find that the lens is an M dwarf star at a distance of 578(+65/-53)pc with a proper motion of 21.39 +/- 0.04 mas/yr. Based on the kinematics and location of this star it more likely to be part of the Galactic thick disk than thin disk population. We confirm that the microlensing event LMC-5 is a jerk-parallax event.
We are using the CTIO 4m Blanco telescope and the MOSAIC imager to carry out a search for microle... more We are using the CTIO 4m Blanco telescope and the MOSAIC imager to carry out a search for microlensing of sources in the LMC. Our goal is to ascertain the nature of the population responsible for the microlensing rate detected by the MACHO collaboration (Alcock et. al ApJ 2000 542, 257). Now in the second year of operation, we will describe our project's status and implementation, including the optimization of exposure times across the LMC. Projections for the anticipated event detection rate are described in the companion talk by Rest (this meeting). This project makes use of the common data analysis pipeline described in the poster by Miceli et. al (this meeting). This project was made possible by a generous grant from the James S. McDonnell foundation.
Proceedings of Spie the International Society For Optical Engineering, Aug 1, 2008
ABSTRACT This paper describes the software systems implemented for the wide-field, automated surv... more ABSTRACT This paper describes the software systems implemented for the wide-field, automated survey telescope, SkyMapper. The telescope is expected to operate completely unmanned and in an environment where failures will remain unattended for several days. Failure analysis was undertaken and the control system extended to cope with subsystem failures, protecting vulnerable detectors and electronics from damage. The data acquisition and control software acquires and stores 512 MB of image data every twenty seconds. As a consequence of the short duty cycle, the preparation of the hardware subsystems for the successive images is undertaken in parallel with the imager readout. A science data pipeline will catalogue objects in the images to produce the Southern Sky Survey.
Astronomy and Astrophysics, Sep 28, 1998
The presence of an anomalously large population of Be stars in the young cluster NGC 330 in the S... more The presence of an anomalously large population of Be stars in the young cluster NGC 330 in the SMC has been noted by , and most recently . We present results from follow-up medium resolution spectra of the bright Be stars identified in and in the spectroscopic study of Mazzali et al. (1996). We find that the study of Mazzali et al. has overestimated the number of Be stars within NGC 330. Many of the bright B type stars identified by Mazzali et al. as Be stars show no signs of emission after correction for the diffuse HII emission pervading the cluster field.
We report pulsations in the X-ray flux of RX J0101.3-7211 in the Small Magellanic Cloud (SMC) wit... more We report pulsations in the X-ray flux of RX J0101.3-7211 in the Small Magellanic Cloud (SMC) with a period of (455±2) s in XMM-Newton EPIC-PN data. The X-ray spectrum can be described by a power-law with a photon index of 0.6±0.1. Timing analysis of ROSAT PSPC and HRI archival data confirms the pulsations and indicates a period increase of ~5 s since 1993. RX J0101.3-7211 varied in brightness during the ROSAT observations with timescales of years with a maximum unabsorbed flux of 6 × 10-13 erg cm-2 s-1 (0.1 - 2.4 keV). The flux during the XMM-Newton observation in the ROSAT band was lower than during the faintest ROSAT detection. The unabsorbed luminosity derived from the EPIC-PN spectrum is 2 × 1035 erg s-1 (0.2 - 10.0 keV) assuming a distance of 60 kpc. Optical spectra of the proposed counterpart taken at the 2.3 m telescope of MSSSO in Australia in August 2000 show strong Hα emission and indicate a Be star. The X-ray and optical data confirm RX J0101.3-7211 as a Be/X-ray binary pulsar in the SMC.
The Astrophysical Journal, Jan 8, 2008
I investigate the discrepancy between the evolution and pulsation masses for Cepheid variables. A... more I investigate the discrepancy between the evolution and pulsation masses for Cepheid variables. A number of recent works have proposed that noncanonical mass loss can account for the mass discrepancy. This mass loss would be such that a 5 M☉ star loses approximately 20% of its mass by arriving at the Cepheid instability strip; a 14 M☉ star, none. Such findings would pose a serious challenge to our understanding of mass loss. I revisit these results in light of the Padova stellar evolutionary models and find evolutionary masses are (17 +/- 5)% greater than pulsation masses for Cepheids between 5 < M/M☉ < 14. I find that mild internal mixing in the main-sequence progenitor of the Cepheid are able to account for this mass discrepancy.
Bulletin of the American Astronomical Society, May 1, 2005
The SkyMapper Telescope is a new facility to replace the Great Melbourne Telescope, destroyed in ... more The SkyMapper Telescope is a new facility to replace the Great Melbourne Telescope, destroyed in the bushfires of 18 Jan 2003. This 1.3m telescope will have an 8 sq degree FOV at a scale of 30 microns per arcsecond, UV-transmissive optics, and be equipped with a 268 million pixel CCD mosaic camera. First light is scheduled for September 2006, with regular operations to commence in January 2007. The telescope will spend 75% of its time undertaking a 6-epoch, 6-band, imaging survey of the 20000 sq-degrees south of declination 0. Known as the "Southern Sky Survey", this program will observe in the Stromgren u, DDO-38, and Sloan g,r,i,z bands, and will cover timescales ranging from hours to years. In addition, a "5-second survey" will be taken under photometric conditions to provide a 6-colour photometric calibration of the southern sky between magnitudes 8 and 15. All photometric information and images will be publically available without proprietary period via a web-accessible database. This work is made possible, in part, by a grant from the Australian Research Council, and through funding by the Australian National University.
Astron J, 2000
We have carried out Wide Field Planetary Camera 2 (WFPC2) F160BW, F555W, and F656N imaging of fou... more We have carried out Wide Field Planetary Camera 2 (WFPC2) F160BW, F555W, and F656N imaging of four young populous clusters: NGC 330, in the Small Magellanic Cloud, and NGC 1818, 2004, and 2100, in the Large Magellanic Cloud. We report photometric results for these four clusters, including identification using photometric colors of the cluster Be star population. We present theoretical WFPC2 and broadband colors and bolometric corrections for LMC and SMC metallicities. The use of the far-UV F160BW filter enables accurate determination of the effective temperatures for stars in the vicinity of the main-sequence turnoff and on the unevolved main sequence. Based on observations with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5-26555.
3D Stellar Evolution, 2003
One of the foremost outstanding problems in the physical sciences is the nature and distribution ... more One of the foremost outstanding problems in the physical sciences is the nature and distribution of the ``dark matter'' that is the gravitationally dominant component of mass in all galaxies, including the Milky Way. One way to search for astrophysical dark matter objects (often called MAssive Compact Halo Objects, or MACHOs) is to search for the transient brightening of background
Ground-based and Airborne Instrumentation for Astronomy, 2006
The Research School of Astronomy and Astrophysics (RSAA) of the Australian National University (A... more The Research School of Astronomy and Astrophysics (RSAA) of the Australian National University (ANU) at Mt Stromlo Observatory is developing a wide-field Cassegrain Imager for the new 1.3m SkyMapper Survey Telescope under construction for Siding Spring Observatory, NSW, Australia. The Imager features a fast-readout, low-noise 268 Million pixel CCD mosaic that provides a 5.7 square degree field of view. Given
Conference Record of the Twenty Sixth IEEE Photovoltaic Specialists Conference - 1997, 1997
The aim of the present work was to optimize a high efficiency process for multicrystalline silico... more The aim of the present work was to optimize a high efficiency process for multicrystalline silicon solar cells (including Al-gettering, oxide and hydrogen passivation, Al-BSF formation, photolithographically defined front metallization) and combine it with the mechanical texturization technique. New cell structures were created, in which only the areas between the front grid are V-grooved. Solar cells were processed on various
Lecture Notes in Computer Science, 1994
While recent hardware and software developments made Geographic Information Systems (GIS) systems... more While recent hardware and software developments made Geographic Information Systems (GIS) systems widely available there are still many problems to be solved. Partially in contrast to the mainstream in GIS research, the most important technical issues of real-world projects point to processes which support an efficient, economic but long living ‘data life cycle’, namely: 1. Data capturing; 2. Continuous map revision, database update and quality checking; 3. Map generalization; 4. Spatial data integration and transfer.
Physics of Atoms and Molecules, 2005
Monthly Notices of the Royal Astronomical Society, 2014
We present a detailed analysis of high-resolution, high S/N spectra for 5 Aquarius stream stars o... more We present a detailed analysis of high-resolution, high S/N spectra for 5 Aquarius stream stars observed with the MIKE spectrograph on the Magellan Clay telescope. Our sample represents one third of the 15 known members in the stream. We find the stream is not mono-metallic: the metallicity ranges from [Fe/H] = −0.63 to −1.58. No anti-correlation in Na-O abundances is present, and we find a strong positive Mg-Al relationship, similar to that observed in the thick disk. We find no evidence that the stream is a result of a disrupted classical globular cluster, contrary to a previously published claim. High [(Na, Ni, α)
Publications of the Astronomical Society of Australia, 2013
One of the major science goals of the SkyMapper Survey of the Southern Hemisphere sky is the dete... more One of the major science goals of the SkyMapper Survey of the Southern Hemisphere sky is the determination of the shape and extent of the halo of the Galaxy. In this paper we quantify the likely efficiency and completeness of the survey as regards the detection of RR Lyrae variable stars, which are excellent tracers of the halo stellar population. We have accomplished this via observations of the RR Lyrae-rich globular cluster NGC 3201. We find that for single epoch uvgri observations followed by two further epochs of g, r imaging, as per the intended three-epoch survey strategy, we recover known RR Lyraes with a completeness exceeding 90%. We also investigate boundaries in the gravity-sensitive single-epoch two-color diagram that yield high completeness and high efficiency (i.e., minimal contamination by non-RR Lyraes) and the general usefulness of this diagram in separating populations.
Lecture Notes in Computer Science, 1999
Digitally capturing and maintaining geospatial data is expensive. The consequence is that there i... more Digitally capturing and maintaining geospatial data is expensive. The consequence is that there is a need for the integration of geoprocessing software into mainstream computer technology and a demand to share geospatial data. This demand includes consequently ...
The Astronomical Journal, 2000
Page 1. THE ASTRONOMICAL JOURNAL, 119 : 1748»1759, 2000 April 2000. The American Astronomical Soc... more Page 1. THE ASTRONOMICAL JOURNAL, 119 : 1748»1759, 2000 April 2000. The American Astronomical Society. All rights reserved. Printed in USA ( WIDE FIELD PLANETARY CAMERA 2 IMAGING OF YOUNG CLUSTERS IN THE MAGELLANIC CLOUDS1 ...
Pedestrian and Evacuation Dynamics 2005, 2007
The ability to collect pedestrian flow data, without the need for subsequent post-processing and ... more The ability to collect pedestrian flow data, without the need for subsequent post-processing and analysis to extract measurements such as density and flow rate is a goal, which up to now, has proved infeasible on a large scale for a number of reasons, such as the cost of processing the data, the ability of the people observing the scene or
We present the results from an analysis of Hubble Space Telescope High Resolution Camera data for... more We present the results from an analysis of Hubble Space Telescope High Resolution Camera data for the Large Magellanic Cloud microlensing event MACHO-LMC-5. By determining the parallax and proper motion of this object we find that the lens is an M dwarf star at a distance of 578(+65/-53)pc with a proper motion of 21.39 +/- 0.04 mas/yr. Based on the kinematics and location of this star it more likely to be part of the Galactic thick disk than thin disk population. We confirm that the microlensing event LMC-5 is a jerk-parallax event.
We are using the CTIO 4m Blanco telescope and the MOSAIC imager to carry out a search for microle... more We are using the CTIO 4m Blanco telescope and the MOSAIC imager to carry out a search for microlensing of sources in the LMC. Our goal is to ascertain the nature of the population responsible for the microlensing rate detected by the MACHO collaboration (Alcock et. al ApJ 2000 542, 257). Now in the second year of operation, we will describe our project's status and implementation, including the optimization of exposure times across the LMC. Projections for the anticipated event detection rate are described in the companion talk by Rest (this meeting). This project makes use of the common data analysis pipeline described in the poster by Miceli et. al (this meeting). This project was made possible by a generous grant from the James S. McDonnell foundation.
Proceedings of Spie the International Society For Optical Engineering, Aug 1, 2008
ABSTRACT This paper describes the software systems implemented for the wide-field, automated surv... more ABSTRACT This paper describes the software systems implemented for the wide-field, automated survey telescope, SkyMapper. The telescope is expected to operate completely unmanned and in an environment where failures will remain unattended for several days. Failure analysis was undertaken and the control system extended to cope with subsystem failures, protecting vulnerable detectors and electronics from damage. The data acquisition and control software acquires and stores 512 MB of image data every twenty seconds. As a consequence of the short duty cycle, the preparation of the hardware subsystems for the successive images is undertaken in parallel with the imager readout. A science data pipeline will catalogue objects in the images to produce the Southern Sky Survey.
Astronomy and Astrophysics, Sep 28, 1998
The presence of an anomalously large population of Be stars in the young cluster NGC 330 in the S... more The presence of an anomalously large population of Be stars in the young cluster NGC 330 in the SMC has been noted by , and most recently . We present results from follow-up medium resolution spectra of the bright Be stars identified in and in the spectroscopic study of Mazzali et al. (1996). We find that the study of Mazzali et al. has overestimated the number of Be stars within NGC 330. Many of the bright B type stars identified by Mazzali et al. as Be stars show no signs of emission after correction for the diffuse HII emission pervading the cluster field.
We report pulsations in the X-ray flux of RX J0101.3-7211 in the Small Magellanic Cloud (SMC) wit... more We report pulsations in the X-ray flux of RX J0101.3-7211 in the Small Magellanic Cloud (SMC) with a period of (455±2) s in XMM-Newton EPIC-PN data. The X-ray spectrum can be described by a power-law with a photon index of 0.6±0.1. Timing analysis of ROSAT PSPC and HRI archival data confirms the pulsations and indicates a period increase of ~5 s since 1993. RX J0101.3-7211 varied in brightness during the ROSAT observations with timescales of years with a maximum unabsorbed flux of 6 × 10-13 erg cm-2 s-1 (0.1 - 2.4 keV). The flux during the XMM-Newton observation in the ROSAT band was lower than during the faintest ROSAT detection. The unabsorbed luminosity derived from the EPIC-PN spectrum is 2 × 1035 erg s-1 (0.2 - 10.0 keV) assuming a distance of 60 kpc. Optical spectra of the proposed counterpart taken at the 2.3 m telescope of MSSSO in Australia in August 2000 show strong Hα emission and indicate a Be star. The X-ray and optical data confirm RX J0101.3-7211 as a Be/X-ray binary pulsar in the SMC.
The Astrophysical Journal, Jan 8, 2008
I investigate the discrepancy between the evolution and pulsation masses for Cepheid variables. A... more I investigate the discrepancy between the evolution and pulsation masses for Cepheid variables. A number of recent works have proposed that noncanonical mass loss can account for the mass discrepancy. This mass loss would be such that a 5 M☉ star loses approximately 20% of its mass by arriving at the Cepheid instability strip; a 14 M☉ star, none. Such findings would pose a serious challenge to our understanding of mass loss. I revisit these results in light of the Padova stellar evolutionary models and find evolutionary masses are (17 +/- 5)% greater than pulsation masses for Cepheids between 5 < M/M☉ < 14. I find that mild internal mixing in the main-sequence progenitor of the Cepheid are able to account for this mass discrepancy.
Bulletin of the American Astronomical Society, May 1, 2005
The SkyMapper Telescope is a new facility to replace the Great Melbourne Telescope, destroyed in ... more The SkyMapper Telescope is a new facility to replace the Great Melbourne Telescope, destroyed in the bushfires of 18 Jan 2003. This 1.3m telescope will have an 8 sq degree FOV at a scale of 30 microns per arcsecond, UV-transmissive optics, and be equipped with a 268 million pixel CCD mosaic camera. First light is scheduled for September 2006, with regular operations to commence in January 2007. The telescope will spend 75% of its time undertaking a 6-epoch, 6-band, imaging survey of the 20000 sq-degrees south of declination 0. Known as the "Southern Sky Survey", this program will observe in the Stromgren u, DDO-38, and Sloan g,r,i,z bands, and will cover timescales ranging from hours to years. In addition, a "5-second survey" will be taken under photometric conditions to provide a 6-colour photometric calibration of the southern sky between magnitudes 8 and 15. All photometric information and images will be publically available without proprietary period via a web-accessible database. This work is made possible, in part, by a grant from the Australian Research Council, and through funding by the Australian National University.
Astron J, 2000
We have carried out Wide Field Planetary Camera 2 (WFPC2) F160BW, F555W, and F656N imaging of fou... more We have carried out Wide Field Planetary Camera 2 (WFPC2) F160BW, F555W, and F656N imaging of four young populous clusters: NGC 330, in the Small Magellanic Cloud, and NGC 1818, 2004, and 2100, in the Large Magellanic Cloud. We report photometric results for these four clusters, including identification using photometric colors of the cluster Be star population. We present theoretical WFPC2 and broadband colors and bolometric corrections for LMC and SMC metallicities. The use of the far-UV F160BW filter enables accurate determination of the effective temperatures for stars in the vicinity of the main-sequence turnoff and on the unevolved main sequence. Based on observations with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5-26555.
3D Stellar Evolution, 2003
One of the foremost outstanding problems in the physical sciences is the nature and distribution ... more One of the foremost outstanding problems in the physical sciences is the nature and distribution of the ``dark matter'' that is the gravitationally dominant component of mass in all galaxies, including the Milky Way. One way to search for astrophysical dark matter objects (often called MAssive Compact Halo Objects, or MACHOs) is to search for the transient brightening of background
Ground-based and Airborne Instrumentation for Astronomy, 2006
The Research School of Astronomy and Astrophysics (RSAA) of the Australian National University (A... more The Research School of Astronomy and Astrophysics (RSAA) of the Australian National University (ANU) at Mt Stromlo Observatory is developing a wide-field Cassegrain Imager for the new 1.3m SkyMapper Survey Telescope under construction for Siding Spring Observatory, NSW, Australia. The Imager features a fast-readout, low-noise 268 Million pixel CCD mosaic that provides a 5.7 square degree field of view. Given
Conference Record of the Twenty Sixth IEEE Photovoltaic Specialists Conference - 1997, 1997
The aim of the present work was to optimize a high efficiency process for multicrystalline silico... more The aim of the present work was to optimize a high efficiency process for multicrystalline silicon solar cells (including Al-gettering, oxide and hydrogen passivation, Al-BSF formation, photolithographically defined front metallization) and combine it with the mechanical texturization technique. New cell structures were created, in which only the areas between the front grid are V-grooved. Solar cells were processed on various
Lecture Notes in Computer Science, 1994
While recent hardware and software developments made Geographic Information Systems (GIS) systems... more While recent hardware and software developments made Geographic Information Systems (GIS) systems widely available there are still many problems to be solved. Partially in contrast to the mainstream in GIS research, the most important technical issues of real-world projects point to processes which support an efficient, economic but long living ‘data life cycle’, namely: 1. Data capturing; 2. Continuous map revision, database update and quality checking; 3. Map generalization; 4. Spatial data integration and transfer.
Physics of Atoms and Molecules, 2005
Monthly Notices of the Royal Astronomical Society, 2014
We present a detailed analysis of high-resolution, high S/N spectra for 5 Aquarius stream stars o... more We present a detailed analysis of high-resolution, high S/N spectra for 5 Aquarius stream stars observed with the MIKE spectrograph on the Magellan Clay telescope. Our sample represents one third of the 15 known members in the stream. We find the stream is not mono-metallic: the metallicity ranges from [Fe/H] = −0.63 to −1.58. No anti-correlation in Na-O abundances is present, and we find a strong positive Mg-Al relationship, similar to that observed in the thick disk. We find no evidence that the stream is a result of a disrupted classical globular cluster, contrary to a previously published claim. High [(Na, Ni, α)
Publications of the Astronomical Society of Australia, 2013
One of the major science goals of the SkyMapper Survey of the Southern Hemisphere sky is the dete... more One of the major science goals of the SkyMapper Survey of the Southern Hemisphere sky is the determination of the shape and extent of the halo of the Galaxy. In this paper we quantify the likely efficiency and completeness of the survey as regards the detection of RR Lyrae variable stars, which are excellent tracers of the halo stellar population. We have accomplished this via observations of the RR Lyrae-rich globular cluster NGC 3201. We find that for single epoch uvgri observations followed by two further epochs of g, r imaging, as per the intended three-epoch survey strategy, we recover known RR Lyraes with a completeness exceeding 90%. We also investigate boundaries in the gravity-sensitive single-epoch two-color diagram that yield high completeness and high efficiency (i.e., minimal contamination by non-RR Lyraes) and the general usefulness of this diagram in separating populations.
Lecture Notes in Computer Science, 1999
Digitally capturing and maintaining geospatial data is expensive. The consequence is that there i... more Digitally capturing and maintaining geospatial data is expensive. The consequence is that there is a need for the integration of geoprocessing software into mainstream computer technology and a demand to share geospatial data. This demand includes consequently ...
The Astronomical Journal, 2000
Page 1. THE ASTRONOMICAL JOURNAL, 119 : 1748»1759, 2000 April 2000. The American Astronomical Soc... more Page 1. THE ASTRONOMICAL JOURNAL, 119 : 1748»1759, 2000 April 2000. The American Astronomical Society. All rights reserved. Printed in USA ( WIDE FIELD PLANETARY CAMERA 2 IMAGING OF YOUNG CLUSTERS IN THE MAGELLANIC CLOUDS1 ...
Pedestrian and Evacuation Dynamics 2005, 2007
The ability to collect pedestrian flow data, without the need for subsequent post-processing and ... more The ability to collect pedestrian flow data, without the need for subsequent post-processing and analysis to extract measurements such as density and flow rate is a goal, which up to now, has proved infeasible on a large scale for a number of reasons, such as the cost of processing the data, the ability of the people observing the scene or