MACHO Photometry Research Articles

Stability of Nonradial Pulsations in the Be Star [MA93]1506

Abstract Period analysis of the 1993–2020 MACHO and OGLE photometry for Be star [MA93]1506 reveals non-radial pulsations are present in all observing seasons, with a mean period of 1.09942 ± 0.00018 days.

Oxygen- and carbon-rich variable red giant populations in the Magellanic Clouds from EROS, OGLE, MACHO, and 2MASS photometry

The carbon-to-oxygen (C/O) ratio of asymptotic giant branch (AGB) stars constitutes an important index of evolutionary and environment/metallicity factor. We develop a method for mass C/O classification of AGBs in photometric surveys without using periods. For this purpose we rely on the slopes in the tracks of individual stars in the colour-magnitude diagram. We demonstrate that our method enables the separation of C-rich and O-rich AGB stars with little confusion. For the Magellanic Clouds we demonstrate that this method works for several photometric surveys and filter combinations. As we rely on no period identification, our results are relatively insensitive to the phase coverage, aliasing, and time-sampling problems that plague period analyses. For a subsample of our stars, we verify our C/O classification against published C/O catalogues. With our method we are able to produce C/O maps of the entire Magellanic Clouds. Our purely photometric method for classification of C- and O-rich AGBs constitutes a method of choice for large, near-infrared photometric surveys. Because our method depends on the slope of colour-magnitude variation but not on magnitude zero point, it remains applicable to objects with unknown distances.

A spectroscopic survey of WNL stars in the Large Magellanic Cloud: general properties and binary status

We report the results of an intense, spectroscopic survey of all 41 late-type, nitrogen-rich Wolf-Rayet (WR) stars in the Large Magellanic Cloud (LMC) observable with ground-based telescopes. This survey concludes the decade-long effort of the Montreal Massive Star Group to monitor every known WR star in the Magellanic Clouds except for the six crowded WNL stars in R136, which will be discussed elsewhere. The focus of our survey was to monitor the so-called WNL stars for radial velocity (RV) variability in order to identify the short- to intermediate-period (P ≤ 200 d) binaries among them. Our results are in line with results of previous studies of other WR subtypes, and show that the binary frequency among LMC WNL stars is statistically consistent with that of WNL stars in the Milky Way. We have identified four previously unknown binaries, bringing the total number of known WNL binaries in the LMC to nine. Since it is very likely that none but one of the binaries is classical, helium-burning WNL star, but rather superluminous, hence extremely massive, hydrogen-burning object, our study has dramatically increased the number of known binaries harbouring such objects, and thus paved the way to determine their masses through model-independent, Keplerian orbits. It is expected that some of the stars in our binaries will be among the most massive known. With the binary status of each WR star now known, we also studied the photometric and X-ray properties of our program stars using archival MACHO photometry as well as Chandra and ROSAT data. We find that one of our presumably single WNL stars is among the X-ray brightest WR sources known. We also identify a binary candidate from its RV variability and X-ray luminosity which harbours the most luminous WR star known in the Local Group.

Long Secondary Periods and Binarity in Red Giant Stars

Observational arguments supporting the binary explanation of the long secondary periods (LSP) phenomenon in red giants are presented. Photometry of about 1200 semiregular variables with the LSP in the Large Magellanic Cloud are analyzed using the MACHO and OGLE photometry. For about 5% of these objects additional ellipsoidal-like or eclipsing-like modulation with the same periods as the LSP is detectable. These double-humped variations are usually shifted in phase comparing to the LSP light curves. I discuss the model of binary system with a red giant as the primary component and a low-mass object as the secondary one. The mass lost by the red giant through the wind follows the spiral pattern in the orbit around the primary star and obscures it causing the LSP variations.

β Cephei and SPB stars in the Large Magellanic Cloud

AbstractThis is a progress report of the study of pulsating main-sequence stars in the LMC. Using the OGLE-II photometry supplemented by the MACHO photometry, we find 64 β Cephei stars in the LMC. Their periods are generally much longer than observed in stars of this type in the Galaxy (the median value is 0.27 d compared with 0.17 d in the Galaxy). In 20 stars with short periods attributable to the β Cephei-type instability, we also find modes with periods longer than ~0.4d. They are likely low-order g modes, which means that in these stars both kinds of variability, β Cephei and SPB, are observed. We also show examples of the multiperiodic SPB stars in the LMC, the first beyond our Galaxy.

A Revised Ephemeris andFUSEObservations of the Supersoft X-Ray Source CAL 83

A new ephemeris has been determined for the supersoft X-ray binary CAL 83 using MACHO photometry. With an improved orbital period of 1.047568 days, it is now possible to phase together photometric and spectroscopic data obtained over the past two decades with new far ultraviolet spectra taken with FUSE. We discuss the properties of the orbital and longterm optical light curves as well as the colors of CAL 83. In the far ultraviolet the only well-detected stellar feature is emission from the O VI resonance doublet. The radial velocity of this emission appears to differ from that of HeII in the optical region, although we only have partial phase coverage for the O VI line. The FUSE continuum variations are similar to the optical light curve in phase and amplitude.

Period-magnitude relations for M giants in Baade's Window NGC 6522

A large and complete sample of stars with K < 9.75 in the NGC 6522 Baade's Window towards the Galactic bulge is examined using light curves extracted from MACHO and IJK photometry from DENIS. The improved statistics resulting from a sample of over 1000 variables allow all four of the sequences A, B, C and D in the KS, log P diagram of the Large Magellanic Cloud to be seen in the Galactic bulge. The bulge sequences, however, show some differences from those in the Magellanic Clouds, possibly due to the effects of higher metallicity. These sequences may have value as distance indicators. A new diagram of the frequency of late-type variables at a given amplitude is derived and compared with the old one from Payne-Gaposchkin. The catalogued semiregular variables of the solar neighbourhood are found to be a subset of the total of semiregulars, biased towards large amplitude.

MACHO photometry of two Large Magellanic Cloud Be X-ray transients, EXO 0531-66 and H 0544-665

Long-term variations are well-known in Be X-ray binaries, and are attributed to non-orbital changes in the structure of the Be circumstellar (equatorial) disc. However, the time-scales involved are so long (tens of days to years) that systematic studies have been very restricted. The ∼8-yr MACHO monitoring of the Large Magellanic Cloud (LMC) therefore presents an ideal opportunity to undertake such studies of Be X-ray systems that lie within the monitored fields. Here we present MACHO observations of two LMC Be X-ray transients, EXO 0531–66 and H 0544–665, the light curves of which show substantial (∼0.5 mag) long-term variations. However, our analysis shows little evidence for any periodic phenomena in the light curves of either source. We find an upper limit for detection of a short (1–100 d) periodicity in the V- and R-band light curves of EXO 0531–66 of 0.041 mag and 0.047 mag semi-amplitude, respectively. The upper limits for the V- and R-band data of H 0544–665 are 0.054 mag and 0.075 mag semi-amplitude, respectively. Both EXO 0531–66 and H 0544–665 become redder as they brighten, possibly due to variations in the structure of the equatorial disc around the Be star. Spectra of both sources show Hα emission; for EXO 0531–66 we find the emission varies over time, thereby confirming its optical identification.

Variable Red Giants in the LMC: Pulsating Stars and Binaries?

AbstractInfrared J and K photometry has been obtained of all red giants in the half square degree region of the LMC bar searched for variables using the MACHO database byWood et al. (1999). The K–log P diagram shows the five distinct period–luminositysequences which are also evident in the MACHO photometry. The K–log P plot allowsidentification of the Mira sequence from among the five observed sequences. As shown in Wood et al. (1999), comparison of observed periods, luminosities and period ratios with theoretical models identifies Miras unambiguously as radial fundamental mode pulsators, while semi-regular variables can be pulsating in the first, second or third overtone, or even the fundamental mode. All these variables lie on just three of the five distinct sequences, and they all appear to be on the AGB. The nature of the variability of stars on the other two sequences is currently unknown. Possibilities include contact and semi-detached binaries, rotating stars with giant star spots, episodic dust formation, rotationally stabilised non-radial g− modes and strange pulsation modes caused by convection–pulsation interaction. These possibilities are discussed.

New R Coronae Borealis Stars in the LMC Discovered in the MACHO Photometry Database

AbstractAn extensive search has been made in the MACHO photometry database for new R Coronae Borealis (RCB) stars in the LMC. Only three such stars were known previously. Nine new LMC RCB stars have been spectroscopically confirmed so far.

RV Tauri Stars and Type II Cepheids in the Magellanic Clouds - Results from the MACHO Database

AbstractA review of the properties of Type II Cepheids and RV Tauri stars in the Magellanic Clouds is presented. In the behaviour of their light and colour curves, the RV Tauri stars appear to be a direct extension of the Type II Cepheids to longer periods. A single P – L – C relationship describes both the Type II Cepheids and RV Tauri stars in the LMC. The derived high intrinsic magnitudes for the RV Tauri variables supports the proposition that these objects are luminous stars evolving off the AGB. Preliminary analysis of the long time-series MACHO photometry indicates one star (MACHO*05:37:45.0–69:54:16) has an obvious ‘period-quadrupled’ periodicity, which is supporting evidence for a period-doubling bifurcation transition to chaotic pulsations.

Calibration of the MACHO Photometry Database

ABSTRACTThe MACHO Project is a microlensing survey that monitors the brightnesses of ∼60 million stars in the Large Magellanic Cloud (LMC), Small Magellanic Cloud, and Galactic bulge. Our database presently contains about 80 billion photometric measurements, a significant fraction of all astronomical photometry. We describe the calibration of MACHO two‐color photometry and transformation to the standard Kron‐Cousins V and R system. Calibrated MACHO photometry may be properly compared with all other observations on the Kron‐Cousins standard system, enhancing the astrophysical value of these data. For ∼9 million stars in the LMC bar, independent photometric measurements of ∼20,000 stars with V≲18 mag in field‐overlap regions demonstrate an internal precision σV = 0.021, σR = 0.019, σV - R = 0.028 mag. The accuracy of the zero point in this calibration is estimated to be ±0.035 mag for stars with colors in the range −0.1 mag<V−R<1.2 mag. A comparison of calibrated MACHO photometry with published photometric sequences and new Hubble Space Telescope observations shows agreement. The current calibration zero‐point uncertainty for the remainder of the MACHO photometry database is estimated to be ±0.10 mag in V or R and ±0.04 mag in V−R. We describe the first application of calibrated MACHO data: the construction of a color‐magnitude diagram used to calculate our experimental sensitivity for detecting microlensing in the LMC.

New R Coronae Borealis Stars in the LMC, Discovered in the MACHO Photometry Database

An extensive search has been made in the MACHO Photometry Database for new R Coronae Borealis (RCB) stars in the LMC. Only three such stars were known previously. Eleven new LMC RCB stars have been spectroscopically confirmed so far.

Is the Large Magellanic Cloud Microlensing Due to an Intervening Dwarf Galaxy?

The recent suggestion that the microlensing events observed toward the Large Magellanic Cloud are due to an intervening Sgr-like dwarf galaxy is examined. A search for foreground RR Lyrae in the MACHO photometry database yields 20 stars whose distance distribution follow the expected halo density profile. Cepheid and red giant branch clump stars in the MACHO database are consistent with membership in the LMC. There is also no evidence in the literature for a distinct kinematic population, for intervening gas, or for the turnoff of such a hypothetical galaxy. We conclude that if the lenses are in a foreground galaxy, it must be a particularly dark galaxy.