Kem Holland Cook

Gravitational Microlensing Events Due to Stellar-Mass Black Holes

We present an analysis of the longest timescale microlensing events discovered by the MACHO Collaboration during a 7 year survey of the Galactic bulge. We find six events that exhibit very strong microlensing parallax signals due, in part, to accurate photometric data from the GMAN and MPS collaborations. The microlensing parallax fit parameters are used in a likelihood analysis, which is able to estimate the distances and masses of the lens objects based on a standard model of the Galactic velocity distribution. This analysis indicates that the most likely masses of five of the six lenses are greater than 1 M☉, which suggests that a substantial fraction of the Galactic lenses may be massive stellar remnants. This could explain the observed excess of long-timescale microlensing events. The lenses for events MACHO-96-BLG-5 and MACHO-98-BLG-6 are the most massive, with mass estimates of M/M☉ = 6 and M/M☉ = 6, respectively. The observed upper limits on the absolute brightness of main-sequence stars for these lenses are less than 1 L☉, so both lenses are black hole candidates. The black hole interpretation is also favored by a likelihood analysis with a Bayesian prior using a conventional model for the lens mass function. We consider the possibility that the source stars for some of these six events may lie in the foreground Galactic disk or in the Sagittarius (Sgr) dwarf galaxy behind the bulge, but we find that bulge sources are likely to dominate our microlensing parallax event sample. Future Hubble Space Telescope observations of these events can either confirm the black hole lens hypothesis or detect the lens stars and provide a direct measurement of their masses. Future observations of similar events by the Space Interferometry Mission or the Keck or VLT interferometers, as explained by Delplancke, Gorski, & Richichi, will allow direct measurements of the lens masses for stellar remnant lenses as well.

The MACHO Project LMC Variable Star Inventory. VII. The Discovery of RV Tauri Stars and New Type II Cepheids in the Large Magellanic Cloud

?????We report the discovery of RV Tauri stars in the Large Magellanic Cloud (LMC). In light- and color-curve behavior, the RV Tauri stars appear to be a direct extension of the type II Cepheids to longer periods. A single period-luminosity-color relationship is seen to describe both the type II Cepheids and the RV Tauri stars in the LMC. We derive the relation V0 = 17.89(?0.20)-2.95(?0.12) log P + 5.49(?0.35)(V - R)0, which is valid for type II Cepheids and RV Tauri stars in the period range 0.9 < log P < 1.75. Assuming a distance modulus to the LMC of 18.5, the relation in terms of the absolute luminosities becomes MV = -0.61(?0.20)-2.95(?0.12) log P + 5.49(?0.35)(V - R)0.

The RR Lyrae Population of the Galactic Bulge from the MACHO Database: Mean Colors and Magnitudes

Mean colors and magnitudes of RR Lyrae stars in 24 fields toward the Galactic bulge from the MACHO database are presented. Accurate mean reddenings are computed for these fields on the basis of the mean colors. The distribution along the line of sight of the RR Lyrae population is examined on the basis of the mean magnitudes, and it is shown that the bulk of the RR Lyrae population is not barred. Only the RR Lyrae stars in the inner fields closer to the Galactic center (l{lt}4{degree}, b{gt}{minus}4{degree}) show evidence for a bar. The red giant clump stars in the MACHO fields, however, clearly show a barred distribution, confirming the results of previous studies. Given the different spatial distribution, the RR Lyrae stars and the clump giants trace two different populations. The RR Lyrae variables would represent the inner extension of the Galactic halo in these fields. {copyright} {ital 1998} {ital The American Astronomical Society}

The MACHO Project LMC Variable Star Inventory. VI. The Second Overtone Mode of Cepheid Pulsation From First/Second Overtone Beat Cepheids

MACHO Project photometry of 45 LMC first and second overtone (FO and SO, respectively) beat Cepheids that pulsate in the FO and SO has been analyzed to determine the light-curve characteristics for the SO mode of Cepheid pulsation. We predict that singly periodic SO Cepheids will have nearly sinusoidal light curves; that we will only be able to discern SO Cepheids from fundamental (F) and FO Cepheids for P1.4 days; and that the SO distribution will overlap the short-period edge of the LMC FO Cepheid period-luminosity relation (when both are plotted as a function of photometric period). We also report the discovery of one SO Cepheid candidate, MACHO*05:03:39.6-70:04:32, with a photometric period of 0.775961?0.000019 days and an instrumental amplitude of 0.047?0.009 mag in V.

Discovery of Five New R Coronae Borealis Stars in the MACHO Galactic Bulge Database

We have identified five new R Coronae Borealis (RCB) stars in the Galactic bulge using the MACHO Project photometry database, raising the total number of known Galactic RCB stars to about 40. We have obtained spectra to confirm the identifications. The fact that four out of the five newly identified RCB stars are cool (T{sub eff} 6000 K) suggests that the preponderance of warm RCB stars among the existing sample is a selection bias. These cool RCB stars are redder and fainter than their warm counterparts and may have been missed in surveys done with blue plates. Based on the number of new RCB stars discovered in the MACHO bulge fields, there may be {approx}250 RCB stars in the reddened exclusion zone toward the bulge.

The Zero Point of Extinction toward Baade's Window from RR Lyrae Stars

We measure the zero point of the Stanek extinction map by comparing the observed (V-K) colors of 20 RR Lyrae stars (type ab) found in the MACHO survey with their intrinsic (V-K)0 colors as a function of period as determined from nearby RR Lyrae stars. We find that the zero point of the Stanek map should be changed by ΔAV = -0.11 ± 0.05 mag, in excellent agreement with the recent measurement of Gould, Popowski, & Terndrup using K giants.

The MACHO project constraints on low mass machos in the galactic halo

The MACHO project has been monitoring about ten million stars in the Large Magellanic Cloud in the search for gravitational microlensing events caused by massive compact halo objects (Machos) in the halo of the Milky Way. The standard analysis for well sampled, long duration microlensing is sensitive to objects with masses 10 −5 ≲ m ≲ 1M ⊙ . However, a different analysis method sensitive to short duration events extends the sensitivity of the experiment to lower masses. Combining the results of the analyses of the first two years of data from the LMC shows that Machos with masses in the range 2.5 × 10 −7 m −2 M ⊙ cannot make up the entire mass of a standard spherical dark halo