arXiv subjects

Patricia Whitelock

Publications and source records attributed to Patricia Whitelock.

16 recordsLinked to original sources

arXiv2024

Global Data in Astronomy: Challenges and Opportunities

Policy Brief on "Global Data in Astronomy: Challenges and Opportunities", distilled from the corresponding panel that was part of the discussions during S20 Policy Webinar on Astroinformatics for Sustainable Development held on 6-7 July 2023. Astronomy is increasingly becoming a data-driven science. Advances in our understanding of the physical mechanisms at work in the Universe require building ever-more sensitive telescopes to gather observations of the cosmos to test and advance our theoretical models of how the universe works. To confront the observed data with our theoretical models we require data hosting, archiving and storage and high-performance computing resources to run the theoretical calculations and compare our simulated and observed universe. We also require the sophisticated development of highly skilled human resources. Newer large projects are often run through international collaborations and partnerships, driving a need for 'open science' and collaborative structure across national boundaries. While astronomical data are useful scientifically, the data do not come with the same ethical/privacy-related restrictions as medical/biological data. Moreover, the ability to use data for new scientific analysis extends and expands the impact and reach of scientific surveys -- this is a strength that national funding agencies should capitalize on. We discuss the management and analysis of such large volumes of data and the corresponding significant challenges that require policy-level preparations. The policy webinar took place during the G20 presidency in India (2023). A summary based on the seven panels can be found here: arxiv:2401.04623.

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arXiv2024

AstroInformatics: Recommendations for Global Cooperation

Policy Brief on "AstroInformatics, Recommendations for Global Collaboration", distilled from panel discussions during S20 Policy Webinar on Astroinformatics for Sustainable Development held on 6-7 July 2023. The deliberations encompassed a wide array of topics, including broad astroinformatics, sky surveys, large-scale international initiatives, global data repositories, space-related data, regional and international collaborative efforts, as well as workforce development within the field. These discussions comprehensively addressed the current status, notable achievements, and the manifold challenges that the field of astroinformatics currently confronts. The G20 nations present a unique opportunity due to their abundant human and technological capabilities, coupled with their widespread geographical representation. Leveraging these strengths, significant strides can be made in various domains. These include, but are not limited to, the advancement of STEM education and workforce development, the promotion of equitable resource utilization, and contributions to fields such as Earth Science and Climate Science. We present a concise overview, followed by specific recommendations that pertain to both ground-based and space data initiatives. Our team remains readily available to furnish further elaboration on any of these proposals as required. Furthermore, we anticipate further engagement during the upcoming G20 presidencies in Brazil (2024) and South Africa (2025) to ensure the continued discussion and realization of these objectives. The policy webinar took place during the G20 presidency in India (2023). Notes based on the seven panels will be separately published.

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arXiv2017

SPIRITS: Uncovering Unusual Infrared Transients With Spitzer

We present an ongoing, systematic search for extragalactic infrared transients, dubbed SPIRITS --- SPitzer InfraRed Intensive Transients Survey. In the first year, using Spitzer/IRAC, we searched 190 nearby galaxies with cadence baselines of one month and six months. We discovered over 1958 variables and 43 transients. Here, we describe the survey design and highlight 14 unusual infrared transients with no optical counterparts to deep limits, which we refer to as SPRITEs (eSPecially Red Intermediate Luminosity Transient Events). SPRITEs are in the infrared luminosity gap between novae and supernovae, with [4.5] absolute magnitudes between -11 and -14 (Vega-mag) and [3.6]-[4.5] colors between 0.3 mag and 1.6 mag. The photometric evolution of SPRITEs is diverse, ranging from < 0.1 mag/yr to > 7 mag/yr. SPRITEs occur in star-forming galaxies. We present an in-depth study of one of them, SPIRITS 14ajc in Messier 83, which shows shock-excited molecular hydrogen emission. This shock may have been triggered by the dynamic decay of a non-hierarchical system of massive stars that led to either the formation of a binary or a proto-stellar merger.

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arXiv2014

Near-Infrared Evidence for a Sudden Temperature Increase in Eta Carinae

Aims. Eta Car's ultra-violet, optical, and X-ray light curves and its spectrum suggest a physical change in its stellar wind over the last decade. It was proposed that the mass-loss rate decreased by a factor of about 2 in the last 15 years. We complement these recent results by investigating the past evolution and the current state of eta Car in the near-infrared (IR). Methods. We present JHKL photometry of eta Car obtained at SAAO Sutherland from 2004-2013 with the Mk II photometer at the 0.75-m telescope and JHKs photometry with SIRIUS at the 1.4-m IRSF telescope from 2012-2013. The near-IR light curves since 1972 are analyzed. Results. The long-term brightening trends in eta Car's JHKL light curves were discontinuous around the 1998 periastron passage. After 1998, the star shows excess emission above the extrapolated trend from earlier dates, foremost in J and H, and the blueward, cyclical progression in its near-IR colors is accelerated. The near-IR color evolution is strongly correlated with the periastron passages. After correcting for the secular trend we find that the color evolution matches an apparent increase in blackbody temperature of an optically thick near-IR emitting plasma component from about 3500 to 6000 K over the last 20 years. Conclusions. We suggest that the changing near-IR emission may be caused by variability in optically thick bremsstrahlung emission. Periastron passages play a key role in the observed excess near-IR emission after 1998 and the long-term color evolution. We thus propose as a hypothesis that angular momentum transfer (via tidal acceleration) during periastron passages leads to sudden changes in eta Car's atmosphere resulting in a long-term decrease in the mass-loss rate.

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arXiv2007

Asymptotic Giant Branch Stars in the Phoenix Dwarf Galaxy

JHKs near-infrared photometry of stars in the Phoenix dwarf galaxy is presented and discussed. Combining these data with the optical photometry of Massey et al. allows a rather clean separation of field stars from Phoenix members. The discovery of a Mira variable (P = 425 days), which is almost certainly a carbon star, leads to an estimate of the distance modulus of 23.10+/-0.18 that is consistent with other estimates and indicates the existence of a significant population of age ~2 Gyr. The two carbon stars of Da Costa have M{bol} = -3.8 and are consistent with belonging to a population of similar age; some other possible members of such a population are identified. A Da Costa non-carbon star is Delta Ks~0.3 mag brighter than these two carbon stars. It may be an AGB star of the dominant old population. The nature of other stars lying close to it in the Ks,(J-Ks) diagram needs studying.

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arXiv2004

Optical Astronomy in Post-Apartheid South Africa: 1994 to 2004

The progress of optical astronomy in post-apartheid South Africa is discussed. Particular emphasis is given to the socio-political climate which embraced the idea of a 10-m class telescope as a flagship project that would lead to widespread development in science, technology and education - not only in South Africa, but across the subcontinent.

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arXiv2002

Luminosities of AGB Variables

The prevailing evidence suggests that most large-amplitude AGB variables follow the period luminosity (PL) relation that has been established for Miras in the LMC and galactic globular clusters. Hipparcos observations indicate that most Miras in the solar neighbourhood are consistent with such a relation. There are two groups of stars with luminosities that are apparently greater than the PL relation would predict: (1) in the LMC and SMC there are large amplitude variables, with long periods, P> 420 days, which are probably undergoing hot bottom burning, but which are very clearly more luminous than the PL relation (these are visually bright and are likely to be among the first stars discovered in more distant intermediate age populations); (2) in the solar neighbourhood there are short period, P<235 days, red stars which are probably more luminous than the PL relation. Similar short-period red stars, with high luminosities, have not been identified in the Magellanic Clouds.

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arXiv2002

The Brightest AGB Stars in the Leo I Dwarf Spheroidal Galaxy

The first results of a study of the the dwarf spheroidal galaxy, Leo I, using the new Japanese-South African Infrared Survey Facility (IRSF) are presented. J,H,K_{s} observations show that most, if not all, of at least the top one magnitude of the AGB is populated by carbon stars. These objects form a tight sequence in the K_{s} - (J-K_{s}) plane. In addition there are five very red objects which are believed to be dust enshrouded AGB stars. They have Mbol near -5.0 and are at the AGB tip. Comparison with carbon Miras in Magellanic Cloud clusters and taking into account other evidence on the ages and metallicities of Leo I populations suggests that these obscured stars belong to the youngest significant population of Leo I and have ages of ~2 Gyr. Some of them can be shown to be variable.

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arXiv2001

Globular Clusters and the Mira Period-Luminosity Relation

A globular cluster distance scale based on Hipparcos parallaxes of subdwarfs has been used to derive estimates of M_K for cluster Miras, including one in the SMC globular cluster NGC121. These lead to a zero-point of the Mira infrared period-luminosity relation, PL(K), in good agreement with that derived from Hipparcos parallaxes of nearby field Miras. The mean of these two estimates together with data on LMC Miras yields an LMC distance modulus of 18.60 +/- 0.10 in evident agreement with a metallicity corrected Cepheid modulus (18.59 +/- 0.10). The use of luminous AGB stars as extragalactic population indicators is also discussed.

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arXiv2001

Dust-enshrouded Asymptotic Giant Branch Stars in the Solar Neighbourhood

A study is made of a sample of 58 dust-enshrouded Asymptotic Giant Branch (AGB) stars (including 2 possible post AGB stars), of which 27 are carbon-rich and 31 are oxygen-rich. These objects were originally identified by Jura & Kleinmann as nearby (within about 1 kpc of the sun) AGB stars with high mass-loss rates, greater than 1E-6 solar masses per year. Ground-based near-infrared photometry, data obtained by IRAS and kinematic data from the literature are combined to investigate the properties of these stars. The light amplitude in the near-infrared is found to be correlated with period, and this amplitude decreases with increasing wavelength. Statistical tests show that there is no reason to suspect any difference in the period distributions of the carbon- and oxygen-rich stars for periods less than 1000 days, and no carbon-rich star has a period longer than 1000 days. The colours are consistent with those of cool stars with evolved circumstellar dust-shells. Luminosities and distances are estimated using a period-luminosity relation. Mass-loss rates, estimated from the 60 micron fluxes, show a correlation with pulsation period and is tightly correlated with the K-[12] colour. The kinematics and scale-height of the sample shows that the sources with periods less than 1000 days must have low mass main-sequence progenitors. It is argued that the three oxygen-rich stars with periods over 1000 days probably had intermediate mass main-sequence progenitors with remaining stars having an average progenitor mass of about 1.3 solar masses. The average lifetime of stars in this phase is estimated to be about 4.0E4 years, indicating they will undergo at most one more thermal pulse before leaving the AGB.

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arXiv2001

An Extremely Slow Nova?

The binary companion to the peculiar F supergiant HD 172481 is shown to be a Mira variable with a pulsation period of 312 days. Its characteristics are within the normal range found for solitary Miras of that period, although its pulsation amplitude and mass-loss rate (M(dot) ~ 3 x 10^{-6} solar masses per year) are higher than average. Reasons are given for suspecting that the F supergiant, which has L ~ 10^4 solar luminosities, is a white dwarf burning hydrogen accreted from its companion.

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arXiv2000

Variability of eta Carinae III

Spectra (1951-78) of the central object in eta Car, taken by A.D. Thackeray, reveal three previously unrecorded epochs of low excitation. Since 1948, at least, these states have occurred regularly in the 2020 day cycle proposed by Damineli et al. They last about 10 percent of each cycle. Early slit spectra (1899-1919) suggest that at that time the object was always in a low state. JHKL photometry is reported for the period 1994-2000. This shows that the secular increase in brightness found in 1972-94 has continued and its rate has increased at the shorter wavelengths. Modulation of the infrared brightness in a period near 2020 days continues. There is a dip in the JHKL light curves near 1998.0, coincident with a dip in the X-ray light curve. Evidence is given that this dip in the infrared repeats in the 2020 day cycle. As suggested by Whitelock & Laney, the dip is best interpreted as an eclipse phenomenon in an interacting binary system; the object eclipsed being a bright region (`hot spot'), possibly on a circumstellar disc or produced by interacting stellar winds. The eclipse coincides in phase and duration with the state of low excitation. It is presumably caused by a plasma column and/or by one of the stars in the system.

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arXiv2000

A Galactic Bar to Beyond the Solar Circle and its Relevance for Microlensing

The Galactic kinematics of Mira variables have been studied using infrared photometry, radial velocities, and Hipparcos parallaxes and proper motions. For Miras in the period range 145 to 200 days (probably corresponding to [Fe/H] in the range -0.8 to -1.3) the major axes of the stellar orbits are concentrated in the first quadrant of Galactic longitude. This is interpreted as a continuation of the bar-like structure of the Galactic Bulge out to the solar circle and beyond.

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arXiv1999

Dust Enshrouded AGB Variables in the LMC

The luminosities and periods of obscured AGB stars in the LMC are discussed using a combination of ISO and ground-based infrared photometry. The bolometric luminosities of these stars fall close to an extrapolation of the period-luminosity relation derived for oxygen-rich Mira variables, with both oxygen- and carbon-rich stars falling close to the same relation. It has been known for many years that there are, in the Magellanic Clouds, significant numbers of large amplitude variables which have considerably higher luminosities than the period-luminosity relation would predict. Many of these can be shown to be undergoing hot bottom burning. It is speculated that all large amplitude AGB stars with luminosities significantly higher than indicated by the period-luminosity relation are undergoing hot bottom burning.

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arXiv1999

Can Mira Variables Tell us the Chemical Abundances in Stellar Systems?

A period-metallicity relation for oxygen-rich Miras derived from globular clusters is applied to similar variables in the galactic Bulge. The metallicity distribution in the Bulge thus obtained is in good agreement with that derived from K giants. This suggests that the periods of Miras are useful indicators of metallicity distributions, at least in old systems such as galactic bulges. For systems such as the LMC the situation is not yet clear.

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arXiv1997

Galactic Kinematics of Cepheids from Hipparcos Proper Motions

The Hipparcos proper motions of 220 galactic Cepheids, together with relevant ground-based photometry, have been analysed. The effects of galactic rotation are very clearly seen. Mean values of the Oort constants, A = 14.82 +/- 0.84 km/s/kpc, and B = -12.37 +/- 0.64 km/s/kpc, and of the angular velocity of circular rotation at the Sun, Omega_0 = 27.19 +/- 0.87 km/s/kpc, are derived. Comparison of the value of A with values derived from recent radial velocity solutions confirm, within the errors, the zero-points of the PL and PLC relations derived directly from the Hipparcos trigonometrical parallaxes of the same stars. The proper motion results suggest that the galactic rotation curve is declining slowly at the solar distance from the galactic centre (dTheta/dR = -2.4 +/- 1.2 km/s/kpc). The component of the solar motion towards the North Galactic Pole is found to be +7.61 +/- 0.64 km/s. Based on the increased distance scale deduced in the present paper the distance to the galactic centre derived in a previous radial velocity study is increased to R_0 = 8.5 +/- 0.5 kpc.

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