arXiv subjects

B. A. Peterson

Publications and source records attributed to B. A. Peterson.

At least 109 records · Page 6Linked to original sources

arXiv1997

The Zero Point of Extinction Toward Baade's Window From RR Lyrae Stars

We measure the zero point of the Stanek (1996) 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 ΔA_V = -0.11 +/- 0.05 mag, in excellent agreement with the recent measurement of Gould, Popowski & Terndrup (1997) using K giants.

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arXiv1997

MACHO Alert 95-30 : First Real-Time Observation of Extended Source Effects in Gravitational Microlensing

We present analysis of MACHO Alert 95-30, a dramatic gravitational microlensing event towards the Galactic bulge whose peak magnification departs significantly from the standard point-source microlensing model. Alert 95-30 was observed in real-time by the Global Microlensing Alert Network (GMAN), which obtained densely sampled photometric and spectroscopic data throughout the event. We interpret the light-curve ``fine structure'' as indicating transit of the lens across the extended face of the source star. This signifies resolution of a star several kpc distant. We find a lens angular impact parameter theta_{min}/theta_{source} = 0.715 +/- 0.003. This information, along with the radius and distance of the source, provides an additional constraint on the lensing system. Spectroscopic and photometric data indicate the source is an M4 III star of radius 61 +/- 12 Rsun, located on the far side of the bulge at 9 kpc. We derive a lens angular velocity, relative to the source, of 21.5 +/- 4.9 km/s/kpc, where the error is dominated by uncertainty in the source radius. Likelihood analysis yields a median lens mass of 0.67{+2.53}{-0.46} Msun, located with 80% probability in the Galactic bulge at a distance of 6.93{+1.56}{-2.25} kpc. If the lens is a main-sequence star, we can include constraints on the lens luminosity. This modifies our estimates to M_lens = 0.53{+0.52}{-0.35} Msun and D_lens = 6.57{+0.99}{-2.25} kpc. Spectra taken during the event show that the absorption line equivalent widths of H alpha and the TiO bands near 6700 A vary, as predicted for microlensing of an extended source. This is most likely due to center-to-limb variation in the stellar spectral lines. These data demonstrate the feasibility of using microlensing limb crossings as a tool to probe stellar atmospheres directly.

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arXiv1996

Planetary Microlensing from the MACHO Project

We present the lightcurves of two microlensing events from the MACHO Project data that are likely to be due to lenses with masses similar to Jupiter's mass. Although the MACHO Project survey data are not sufficient to definitively establish the identification of planetary mass lenses in these cases, observations by microlensing follow-up networks such as GMAN and PLANET should be able to definitively determine the planetary nature of similar events which may occur in the near future.

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arXiv1996

Gravitational Microlensing Results from MACHO

The MACHO project is searching for dark matter in the form of massive compact halo objects (Machos), by monitoring the brightness of millions of stars in the Magellanic Clouds to search for gravitational microlensing events. Analysis of our first 2.3 years of data for 8.5 million stars in the LMC yields 8 candidate microlensing events, well in excess of the $\approx 1$ event expected from lensing by known low-mass stars. The event timescales range from 34 to 145 days, and the estimated optical depth is $\sim$ 2 x 10^{-7}, about half of that expected from a `standard' halo. Likelihood analysis indicates the typical lens mass is $0.5^{+0.3}_{-0.2} \Msun$, suggesting they may be old white dwarfs.

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arXiv1996

Cepheids in the LMC: Results from the MACHO Project

The approximately 1500 Cepheid variables in 22 LMC fields of the MACHO Project survey have been calibrated and analysed. In this paper, we report improved period ratios for a total of 73 beat Cepheids and provide a first look at the Fourier decomposition parameters for both singly- and doubly-periodic Cepheids. We also note an unusual amplitude-changing pulsator.

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arXiv1996

RR Lyrae Stars in the MACHO Database

The MACHO Project has catalogued $\sim 8000$ RR Lyrae stars in the Large Magellanic Cloud, $\sim 1800$ in the Galactic bulge, and $\sim$ 50 in the Sgr dwarf galaxy. These variables are excellent distance indicators, and are used as tools to study the structure of the Large Magellanic Cloud and the bulge. The large datasets also probe uncommon pulsation modes. A number of double-mode RR Lyrae stars (RRd) are found in the Large Magellanic Cloud sample. These stars provide important clues for understanding the formation and evolution of the inner Galaxy, the Large Magellanic Cloud and the Sgr dwarf galaxy. A large number of second overtone pulsators (RRe) are found in the LMC and bulge. Finally, the RR Lyrae belonging to the Sgr dwarf yield an accurate distance to this galaxy. Their presence also alerts us of the very interesting possibility of distant sources for bulge microlensing events.

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arXiv1996

Colors and K-Band Counts of Extremely Faint Field Galaxies

We combine deep K-band (Keck) with V- and I-band (NTT) observations of two high-Galactic latitude fields, surveying a total of ~2 sq. arcmin. The K-band galaxy counts continue to rise above K=22, reaching surface densities of few x 10^5 per sq. degree. The slope for the counts is (d log(N) per mag per sq. degree) = 0.23 +/- 0.02 between K=18-23, consistent with other deep K surveys. The numbers of galaxies in each mag bin is about two times greater than the galaxy counts of Djorgovski et al. (1995). The optical and near infrared magnitudes of all objects detected in the V+I+K image are discussed in the context of grids of isochrone synthesis galaxy evolutionary models (Bruzual & Charlot 1993, 1995). The colors of most of the observed galaxies are consistent with a population drawn from a broad redshift distribution. A few galaxies at K=19-20 are red in both colors (V-I>3; I-K>2), consistent with being early-type galaxies having undergone a burst of star formation at z>5 and viewed at z~1. At K>20, we find ~8 ``red outlier'' galaxies with I-K>4 and V-I<2.5, whose colors are difficult to mimic by a single evolving or non-evolving stellar population at any redshift. They are likely either low-metallicity, dusty dwarf galaxies, or old galaxies at high redshift (z>1.2). Their surface density is several per square arcminute, which is so high that they are probably common objects of low luminosity $L<L_*$.

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arXiv1996

The Parkes Half-Jansky Flat-Spectrum Sample

We present a new sample of Parkes half-Jansky flat-spectrum radio sources having made a particular effort to find any previously unidentified sources. The sample contains 323 sources selected according to a flux limit of 0.5 Jy at 2.7 GHz, a spectral index measured between 2.7 and 5.0 GHz of alpha > -0.5 (where S(v) is proportional to v to the power of alpha), Galactic latitude abs(b) > 20 deg and -45 deg < Declination (B1950) < +10 deg. The sample was selected from a region 3.90 steradians in area. We have obtained accurate radio positions for all the unresolved sources in this sample and combined these with accurate optical positions from digitised photographic sky survey data to check all the optical identifications. We report new identifications based on \R- and \Kn-band imaging and new spectroscopic measurements of many of the sources. We present a catalogue of the 323 sources of which 321 now have identified optical counterparts and 277 have measured spectral redshifts.

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arXiv1996

The MACHO Project LMC Variable Star Inventory IV: Multimode RR Lyrae Stars, Distance to the LMC and Age of the Oldest Stars

We report the discovery of 73 double-mode RR Lyrae (RRd) stars in fields near the bar of the LMC. The stars are detected among the MACHO database of short-period variables that currently contains about 7900 RR Lyrae stars. Fundamental periods (P_0) for these stars are found in the range 0.46-0.55 days and first overtone-to-fundamental period ratios are found to be in the range 0.742 < P_1/P_0 < 0.748. A significant fraction of our current sample have period ratios smaller than any previously discovered RRd variables. We present mean magnitudes, colors, and lightcurve properties for all LMC RRd stars detected to date. The range in period ratios is unexpectedly large. We present a determination of absolute magnitudes for these stars based primarily on pulsation theory and the assumption that all observed stars are at the fundamental blue edge (FBE) of the instability strip. Comparison of the calibrated MACHO V and R_KC photometry with these derived absolute magnitudes yields an absorption-corrected distance modulus to the LMC of 18.57 +/- 0.19 mag which is in good agreement with that found (18.5) through comparison of galactic and LMC Cepheids. Adopting this luminosity calibration, we derive an increase in the distance modulus, and thus a reduction in the age found via isochrone fitting for M15 of about 33% and discuss the implications for cosmology.

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arXiv1996

The MACHO Project LMC Microlensing Results from the First Two Years and the Nature of the Galactic Dark Halo

The MACHO Project is a search for dark matter in the form of massive compact halo objects (Machos). Photometric monitoring of millions of stars in the Large Magellanic Cloud (LMC), Small Magellanic Cloud (SMC), and Galactic bulge is used to search for gravitational microlensing events caused by these otherwise invisible objects. Analysis of the first 2.1 years of photometry of 8.5 million stars in the LMC reveals 8 candidate microlensing events. This is substantially more than the number expected ($\sim 1.1$) from lensing by known stellar populations. The timescales ($\that$) of the events range from 34 to 145 days. We estimate the total microlensing optical depth towards the LMC from events with $2 < \that < 200$ days to be $τ_2^{200} = 2.9 ^{+1.4}_{-0.9} \ten{-7}$ based upon our 8 event sample. This exceeds the optical depth, $τ_{\rm backgnd} = 0.5 \ten{-7}$, expected from known stars, and the difference is to be compared with the optical depth predicted for a ``standard" halo composed entirely of Machos: $τ_{halo} = 4.7\ten{-7}$. Likelihood analysis gives a fairly model independent estimate of the halo mass in Machos within 50 kpc of $2.0^{+1.2}_{-0.7} \ten{11} \msun$, about half of the ``standard halo" value. We also find a most probable Macho mass of $0.5^{+0.3}_{-0.2}\msun$, although this value is strongly model dependent. Additionally, the absence of short duration events places stringent upper limits on the contribution of low-mass Machos: objects from $10^{-4} \msun$ to $0.03 \msun$ contribute $\simlt 20\%$ of the ``standard" dark halo.

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arXiv1996

The MACHO Project 2nd Year LMC Microlensing Results and Dark Matter Implications

The MACHO Project is searching for galactic dark matter in the form of massive compact halo objects (Machos). Millions of stars in the Large Magellanic Cloud (LMC), Small Magellanic Cloud (SMC), and Galactic bulge are photometrically monitored in an attempt to detect rare gravitational microlensing events caused by otherwise invisible Machos. Analysis of two years of photometry on 8.5 million stars in the LMC reveals 8 candidate microlensing events, far more than the $\sim1$ event expected from lensing by low-mass stars in known galactic populations. From these eight events we estimate the optical depth towards the LMC from events with $2 < \that < 200$ days to be $τ_2^{200} \approx 2.9 ^{+1.4}_{-0.9} \ten{-7}$. This exceeds the optical depth of $0.5\ten{-7}$ expected from known stars and is to be compared with an optical depth of $4.7\ten{-7}$ predicted for a ``standard'' halo composed entirely of Machos. The total mass in this lensing population is $\approx 2^{+1.2}_{-0.7} \ten{11} \msun$ (within 50 kpc from the Galactic center). Event timescales yield a most probable Macho mass of $0.5^{+0.3}_{-0.2}\msun$, although this value is quite model dependent.

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arXiv1996

A Binary Lensing Event Toward the LMC: Observations and Dark Matter Implications

The MACHO collaboration has recently analyzed 2.1 years of photometric data for about 8.5 million stars in the Large Magellanic Cloud (LMC). This analysis has revealed 8 candidate microlensing events and a total microlensing optical depth of $τ_{meas} = 2.9 +1.4/-0.9 \times 10^{-7}$. This significantly exceeds the number of events (1.1) and the microlensing optical depth predicted from known stellar populations: $τ_{back} = 5.4\times 10^{-8}$, but it is consistent with models in which about half of the standard dark halo mass is composed of Machos of mass $\sim 0.5 \msun$. One of these 8 events appears to be a binary lensing event with a caustic crossing that is partially resolved which allows us to estimate the distance to the lenses. If the source star is not a short period binary star, then we show that the lens system is very likely to reside in the LMC. However, if we assume that the optical depth for LMC-LMC lensing is large enough to account for our entire lensing signal, then the binary event does not appear to be consistent with lensing of a single LMC source star by a binary residing in the LMC. Thus, while the binary lens may indeed reside in the LMC, there is no indication that most of the lenses reside in the LMC.

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arXiv1996

MACHO Project Photometry of RR Lyrae Stars in the Sgr Dwarf Galaxy

We report the discovery of 30 type a,b RR Lyrae (RRab) which are likely members of the Sagittarius (Sgr) dwarf galaxy. Accurate positions, periods, amplitudes and magnitudes are presented. Their distances are determined with respect to RRab in the Galactic bulge found also in the MACHO 1993 data. For R$_{\odot} = 8$ kpc, the mean distance to these stars is $D = 22 \pm 1$ kpc, smaller than previous determinations for this galaxy. This indicates that Sgr has an elongated main body extending for more than 10 kpc, which is inclined along the line of sight, with its northern part (in Galactic coordinates) closer to us. The size and shape of Sgr give clues about the past history of this galaxy. If the shape of Sgr follows the direction of its orbit, the observed spatial orientation suggests that Sgr is moving away from the Galactic plane. Also, Sgr stars may be the sources of some of the microlensing events seen towards the bulge.

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arXiv1996

The MACHO Project LMC Variable Star Inventory: III. New R Coronae Borealis Stars

We report the discovery of two new R Coronae Borealis (RCB) stars in the Large Magellanic Cloud (LMC) using the MACHO project photometry database. The identification of both stars has been confirmed spectroscopically. One is a cool RCB star (T_eff about 5000 K) characterized by very strong Swan bands of C_2 and violet bands of CN, and weak or absent Balmer lines, G-band and 12C-13C bands. The second star is an example of a hot RCB star of which only 3 were previously known to exist in the Galaxy and none in the LMC. Its spectrum is characterized by several C II lines in emission. Both stars have shown deep declines of Delta V > 4 mag in brightness. The new stars are significantly fainter at maximum light than the three previously known LMC RCB stars. The amount of reddening toward these stars is somewhat uncertain but both seem to have absolute magnitudes, M_V, about half a magnitude fainter than the other three stars. Estimates of M_Bol find that the hot RCB star lies in the range of the other three stars while the cool RCB star is fainter. The two cool LMC RCB stars are the faintest at M_Bol. The discovery of these two new stars brings to five the number of known RCB stars in the LMC and demonstrates the utility of the MACHO photometric database for the discovery of new RCB stars.

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arXiv1996

The MACHO Project: Limits on Planetary Mass Dark Matter in the Galactic Halo from Gravitational Microlensing

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. In our standard analysis, we have searched this data set for well sampled, long duration microlensing lightcurves, detected several microlensing events consistent with Machos in the 0.1 < m < 1.0 M_sun mass range, and set limits on the abundance of objects with masses 1e-5 < m < 0.1 M_sun. In this paper, we present a different type of analysis involving the search for very short time scale brightenings of stars which is used to set strong limits on the abundance of lower mass Machos. Our analysis of the first two years of data toward the LMC indicates that Machos with masses in the range 2.5e-7 < m < 5.2e-4 M_sun cannot make up the entire mass of a standard spherical dark halo. Combining these results with those from the standard analysis, we find that the halo dark matter may not be comprised of objects with masses 2.5e-7 < m < 8.1e-2 M_sun.

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arXiv1996

The Stromlo-APM Redshift Survey III. Redshift Space Distortions, Omega and Bias

Galaxy redshift surveys provide a distorted picture of the universe due to the non-Hubble component of galaxy motions. By measuring such distortions in the linear regime one can constrain the quantity $β= Ω^{0.6}/b$ where $Ω$ is the cosmological density parameter and $b$ is the (linear) bias factor for optically-selected galaxies. In this paper we apply two techniques for estimating $β$ from the Stromlo-APM redshift survey --- (1) measuring the anisotropy of the redshift space correlation function in spherical harmonics and (2) comparing the amplitude of the direction-averaged redshift space correlation function to the real space correlation function. We test the validity of these techniques, particularly whether the assumption of linear theory is justified, using two sets of large $N$-body simulations. We find that the first technique is affected by non-linearities on scales up to $\sim 30 \hMpc$. The second technique is less sensitive to non-linear effects and so is more useful for existing redshift surveys. The Stromlo-APM survey data favours a low value for $β$, with $β\simlt 0.6$. A bias parameter $b \approx 2$ is thus required if $Ω\equiv 1$. However, higher-order correlations measured from the APM galaxy survey (Gaztañaga and Frieman 1994) indicate a low value for the bias parameter $b \approx 1$, requiring that $Ω\simlt 0.5$. We also measure the relative bias for samples of galaxies of various luminosity and morphological type and find that low-luminosity galaxies are roughly three times less biased than $L^*$ galaxies. For the galaxy population as a whole, we measure a real space variance of galaxy counts in $8 \hMpc$ spheres of $(σ^2_8)_g = 0.89 \pm 0.05$.

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arXiv1995

The MACHO Project: 45 Candidate Microlensing Events from the First Year Galactic Bulge Data

We report the detection of 45 candidate microlensing events in fields toward the Galactic bulge. These come from the analysis of 24 fields containing 12.6 million stars observed for 190 days in 1993. Many of these events are of extremely high signal to noise and are remarkable examples of gravitational microlensing. The distribution of peak magnifications is shown to be consistent with the microlensing interpretation of these events. Using a sub-sample of 1.3 million ``Clump Giant" stars whose distance and detection efficiency are well known, we find 13 events and estimate the microlensing optical depth toward the Galactic Bulge as $τ_{\rm bulge} = 3.9 {+ 1.8 \atop - 1.2} \times 10^{-6}$ averaged over an area of $\sim 12$ square degrees centered at Galactic coordinates $\ell = 2.55^\circ$ and $b = -3.64^\circ$. This is similar to the value reported by the OGLE collaboration, and is marginally higher than current theoretical models for $τ_{\rm bulge}$. The optical depth is also seen to increase significantly for decreasing $\vert b\vert$. These results demonstrate that obtaining large numbers of microlensing events toward the Galactic bulge is feasible, and that the study of such events will have important consequences for the structure of the Galaxy and its dark halo.

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arXiv1995

The MACHO Project Dark Matter Search

We provide a status report on our search for dark matter in our Galaxy in the form of massive compact halo objects (or Machos), using gravitational microlensing of background stars. This search uses a very large format CCD camera on the dedicated 1.27m telescope at Mt.~Stromlo, Australia, and has been taking data for almost 3 years. At present, we are in the midst of analyzing our second year data for 8 million stars in the Large Magellanic Cloud. We find more microlensing events than expected from known stellar populations suggesting that Machos are indeed present in the Galactic halo, but the observed microlensing rate toward the LMC is too small to allow for a halo dominated by sub-stellar Machos. Our observations of the Galactic bulge have also yielded substantially more microlensing events than anticipated including a number of exotic ``deviant" microlensing events. The implications of these results are discussed.

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