MACHO Collaboration Research Articles

Contribution of Brown Dwarfs and White Dwarfs to Recent Microlensing Observations and to the Halo Mass Budget

We examine the recent results of the MACHO collaboration toward the Large Magellanic Cloud in terms of a halo brown dwarf or white dwarf population. The possibility that most of the microlensing events are due to brown dwarfs is totally excluded by large-scale kinematic properties. The white dwarf scenario is examined in detail in the context of the most recent white dwarf cooling theory which explicitly takes into account the extra energy source that derives from the carbon-oxygen differentiation at crystallization and the subsequent Debye cooling. We show that the observational constraints arising from the luminosity function of high-velocity white dwarfs in the solar neighborhood and from the recent Hubble Space Telescope deep field counts are consistent with a white dwarf contribution to the halo missing mass as large as 50%, provided there is (1) an initial mass function strongly peaked around ~1.7 M☉, and (2) a halo age older than ~18 Gyr.

Limits on the Halo White Dwarf Component of Baryonic Dark Matter from the Hubble Deep Field

The MACHO collaboration lensing event statistics suggest that a significant fraction of the dark Galactic halo can be comprised of baryonic matter in the form of white dwarf stars with masses between 0.1 and 1.0 M☉. Such a halo white dwarf population, in order to have escaped detection by those who observe the white dwarf luminosity function of the disk, must have formed from an old population. The observations indicate that the number of halo white dwarfs per cubic parsec per unit bolometric magnitude is less than 10-5 at 10-4.5 L☉; the number must rise significantly at lower luminosities to provide the needed baryonic halo mass. Such white dwarfs may easily escape detection in most current and earlier surveys. Though it is limited in angular extent, the Hubble Deep Field (HDF) probes a sufficient volume of the Galactic halo to provide interesting limits on the number of halo white dwarf stars and on the fraction of the halo mass that they can make up. If the HDF field can be probed for stars down to V = 29.8 then the MACHO result suggests that there could be up to 12 faint halo white dwarfs visible in the HDF. Finding (or not finding) these stars in turn places interesting constraints on star formation immediately following the formation of the galaxy.

Gravitational Microlensing by Clustered MACHOs

It has been proposed that the MACHOs in our galaxy could be clumped in globular cluster--like associations or RAMBOs (robust associations of massive baryonic objects) (Moore \& Silk 1995). Here we investigate the effect such clustering has on the microlensing of stars in the Large Magellanic Cloud. We find that the lensing in a one square degree field could be dominated by just a few clusters. As a result the lensing properties vary widely depending on the position and velocity of those clusters which happen to lie between us and the LMC. Moreover we find a large variance in time scale distributions that suggests that the small number statistics could easily be dominated by events in the tails of the unclustered distribution ({\it e.g.} by long periods). We compare our results with the MACHO collaboration's data and find that a ``standard'' halo made entirely of MACHOs is not strongly disfavored if the clusters have masses of $10^6 M_{\odot}$. For less massive clusters such a halo is not as likely. For $10^4 M_{\odot}$ clusters the microlensing statistics are essentially unchanged from the unclustered case. It may be possible to detect very massive clusters from the distribution of events in time-scale and space. We provide some example time- scale distributions.

Real-Time Detection and Multisite Observations of Gravitational Microlensing

The MACHO collaboration is carrying out an extensive survey to detect gravitational microlensing. We have recently demonstrated the ability to detect candidate microlensing events in real time, often well before the peak amplification of the event. This has made possible the acquisition of spectra over the course of an event, the invariance of which has lent strong support to the microlensing interpretation. This paper reports on photometric data that were acquired from two sites, Cerro Tololo Inter-American Observatory and Mount Stromlo, in response to the real time detection of a microlensing event. The superior photometry obtained from Chile and the complementary time coverage demonstrate the viability of mounting an aggressive campaign of microlensing follow-up observations.

Monte Carlo Simulations of MACHO Parallaxes from a Satellite

Three ongoing microlensing experiments have found more candidate events than expected from the known stars. These experiments measure only one parameter of the massive compact halo objects (machos), the magnification time scale of the events. More information is required to understand the nature of the machos. A satellite experiment has been proposed to measure their projected transverse speed $\tilde{v} = v/(1-z)$, where $v$ is the macho transverse speed and $z$ its distance divided by the distance of the source. Measurement of $\tilde{v}$ would determine whether the machos were in the Galactic disk, Galactic halo, or in the Large Magellanic Cloud (LMC). We simulate events observed toward the LMC by the Earth and by a satellite in an Earth like heliocentric orbit. To leading order, such an experiment determines $\tilde{v}$ up to a two fold degeneracy. More precise measurements break the degeneracy. We show that with photometric precisions of 3\% to 4\% and approximately 1 observation per day, $\tilde{v}$ can be measured with a maximum error of 20\% for 70\% to 90\% of events similar to the ones reported by the EROS and MACHO collaborations. The projected transverse velocity is known with the same maximum error for 60\% to 75\% of these events. This 20\% maximum error is not a 1 $\sigma$ error but is mostly due to degeneracy between two possible solutions, each one being localized to much better than 20\%. These results are obtained with an Earth-satellite separation of 1 AU, and are improved by a larger separation.

Are Microlensing Events Contaminated by Dwarf Nova Eruptions?

We present a simple test that can distinguish between the light curves of candidate microlensing events and those of some dwarf nova eruptions that have a similar morphology. We find that the distribution of the amplitudes versus durations of ordinary dwarf nova eruptions is significantly different from that exhibited by the main body of 53 microlensing candidates discovered by the MACHO and OGLE collaborations. The most serious source of potential contamination may be due to old novae that exhibit dwarf nova outbursts. By adopting the current estimates for the space density of such old novae, we find that a significant fraction of the microlensing candidates could be cataclysmic variables.

A Self-Consistent Dynamical Model for the COBE Observed Galactic Bar and Its Application to Microlensing

AbstractA self-consistent stellar dynamical model for the Galactic bar is constructed from about 500 numerically computed orbits with an extension of the Schwarzschild technique. The model fits the COBE found asymmetric boxy light distribution and the observed stellar kinematics of the bulge. The model potential is also consistent with the non-circular motions of the HI and CO velocity maps of the inner Galaxy. We also use the stellar bar model to construct an N-body model to study stability and a microlensing map towards the bulge, which can account for the observed optical depth and the event duration by the MACHO and OGLE collaborations. The technique used here can be applied to interpret light and velocity data of external bulges/bars and galactic nuclei.

Real-Time Detection Of Gravitational Microlensing

Real-time detection of microlensing has moved from proof of concept in 1994 (Udalski et al. 1994a, Alcock et al. 1994) to a steady stream of events this year. Global dissemination of these events by the MACHO and OGLE collaborations has made possible intensive photometric and spectroscopic follow up from widely dispersed sites confirming the microlensing hypothesis (Benetti 1995). Improved photometry and increased temporal resolution from follow up observations greatly increases the possibility of detecting deviations from the standard point-source, point-lens, inertial motion microlensing model. These deviations are crucial in understanding individual lensing systems by breaking the degeneracy between lens mass, position and velocity. We report here on GMAN (Global Microlensing Alert Network), the coordinated follow up of MACHO alerts.

Interpreting the candidate Galactic microlensing events

Four microlensing collaborations are presently searching for compact matter in the Galaxy and all have detected possible candidates. Using the detection efficiencies recently published by the MACHO and OGLE collaborations, we present Monte-Carlo calculations of the expected optical depth, rates and timescales, along the LMC and Galactic bulge lines of sight, for dark matter in a four-component `standard Galaxy' model with a spherically-symmetric halo and spheroid. Using the typically observed event durations we show that, whilst the halo fraction comprised of compact matter is likely to be $f_{h} < 0.4$, a `no halo compact matter' hypothesis is ruled out at greater than the 80\% confidence level, unless the LMC itself has a substantial halo of such objects. On the basis of the timescales observed by OGLE towards the bulge we find the rate predicted by the model to be in good agreement with the number of OGLE detections. We compute lens mass probability distributions for the various components and compare these estimates with current observational and theoretical constraints on the mass scale of baryonic dark matter. We assess the uniformity of the amplification distributions for the published EROS, MACHO and OGLE events and find that they are quite consistent with the microlensing hypothesis, although the OGLE candidate selection criteria mean that its data are particularly sensitive to photometric selection effects. The EROS team has recently placed strong limits on the density contribution of very low mass halo objects from their short timescale CCD search. On the basis of a recent study of the flux amplification of finite-size sources by Simmons, Newsam \& Willis \shortcite{simm95} we suggest that EROS may have detected up to 5 low-amplification events due to halo lenses with mass $m\sim 10^{-7}~\sm$.

A case for a baryonic dark halo.

Recent observations of microlensing events in the Large Magellanic Cloud by the MACHO and EROS collaborations suggest that an important fraction of the galactic halo is in the form of Massive Halo Objects (MHO) with mass $\sim 0.1 M_{\odot}$. We outline a scenario in which dark clusters of MHO with mass $\sim 0.1 M_{\odot}$ and $H_2$ molecular clouds form in the halo at galactocentric distances larger than $\sim$ 10 - 20 kpc, provided baryons are a major constituent of the halo. Possible signatures of our picture are discussed.

A Dual CCD Mosaic Camera System Searching for Massive Compact Halo Objects (Machos)

The Macho Collaboration uses a dedicated 1.27-m telescope (The Great Melbourne Telescope) at Mount Stromlo to make photometric measurements of tens of millions of stars per night searching for the gravitational microlensing signature of MACHOs in the halo and disk of the Milky Way. A prime focus corrector and dichroic beamsplitter provide red (6300–7800 å) and blue (4500–6300 å) foci with one degree fields. A two by two mosaic of 2048 × 2048 pixel CCDs in each focal plane provides simultaneous images of 0.5 square degrees. By August of 1994, more than 20,000, 32 megapixel images will have been collected of fields in the Large Magellanic Cloud (LMC), Small Magellanic Cloud and the bulge of the Milky Way. We have implemented an online analysis system which produces photometric reductions of a night's data (five Gbyte of images) within 24 hours. This system allows us to identify and follow interesting events in real-time. In this search, we have identified more than 60,000 variable stars, and a preliminary analysis of their types and distribution will be presented. Microlensing events toward the LMC and the bulge have been discovered and detection efficiencies are being calculated to constrain the MACHO content of the Milky Way's halo.

A Dual CCD Mosaic Camera System Searching for Massive Compact Halo Objects (Machos)

The Macho Collaboration uses a dedicated 1.27-m telescope (The Great Melbourne Telescope) at Mount Stromlo to make photometric measurements of tens of millions of stars per night searching for the gravitational microlensing signature of MACHOs in the halo and disk of the Milky Way. A prime focus corrector and dichroic beamsplitter provide red (6300–7800 å) and blue (4500–6300 å) foci with one degree fields. A two by two mosaic of 2048 × 2048 pixel CCDs in each focal plane provides simultaneous images of 0.5 square degrees. By August of 1994, more than 20,000, 32 Megapixel images will have been collected of fields in the Large Magellanic Cloud (LMC), Small Magellanic Cloud and the bulge of the Milky Way. We have implemented an online analysis system which produces photometric reductions of a night's data (five Gbyte of images) within 24 hours. This system allows us to identify and follow interesting events in real-time. In this search, we have identified more than 60,000 variable stars, and a preliminary analysis of their types and distribution will be presented. Microlensing events toward the LMC and the bulge have been discovered and detection efficiencies are being calculated to constrain the MACHO content of the Milky Way's halo.

Variable Stars in the MACHO Collaboration Database

AbstractThe MACHO Collaboration’s search for baryonic dark matter via its gravitational microlensing signature has generated a massive database of time ordered photometry of millions of stars in the LMC and the bulge of the Milky Way. The search’s experimental design and capabilities are reviewed and the dark matter results are briefly noted. Preliminary analysis of the ~ 39,000 variable stars discovered in the LMC database is presented and examples of periodic variables are shown. A class of a periodically variable Be star is described which is the closest background to microlensing which has been found. Plans for future work on variable stars using the MACHO data are described.

Models of the Double-mode Cepheids in the Large Magellanic Cloud

AbstractVery recently the MACHO Collaboration published detailed data for 45 double-mode Cepheids in the Large Magellanic Cloud. Here we compare these data with new stellar pulsation models. We conclude that the new period ratios are in good agreement with a metal content Z = 0.005 – 0.01 in the Large Magellanic Cloud.

Magnification bias in galactic microlensing searches

It is shown that a significant amount of detectable gravitational microlensing events that could potentially be found by Massively Parallel Photometry (MAPP) project (such as the MACHO, EROS, and OGLE collaborations) will occur for stars too dim to be easily noticed individually by these projects. This is the result of a large magnification bias effect, a bias of including high-magnification events in any flux-limited sample. The probablility of detecting these events may be as high as 2.3 times the lensing probability of stars currently being monitored by MAPP collaborations.

On the mass of the dark compact objects in the Galactic disk

Recently the Polish-American collaboration OGLE has reported the observation of four possible microlensing events by monitoring, over several months, the brightness of millions of stars in the region of the galactic bulge. If these events are due to microlensing, the most accurate way to get information on the mass of the dark compact objects, that acted as gravitational lenses, is to use the method of the mass moments. Here I apply this method to the analysis of the events detected by OGLE. The average mass turns out to be 0.28$M_{\odot}$, suggesting that the lens objects are faint disk stars. The same method applied to the five microlensing events detected so far by the EROS and MACHO collaborations, which monitor stars in the Large Magellanic Cloud, leads to an average value of 0.08$M_{\odot}$ for the dark compact halo objects.

Whither do the microlensing agents rove?

The EROS and MACHO collaborations have reported observations of light curves of stars in the Large Magellanic Cloud that are compatible with gravitational microlensing by intervening massive objects, presumably Brown-Dwarf stars. The OGLE and MACHO teams have also seen similar events in the direction of the galactic Bulge. Current data are insufficient to decide whether the Brown-Dwarfs are dark-matter constituents of the non-luminous galactic Halo, or belong to a more conventional population, such as that of faint stars in the galactic Spheroid, in its Thin or Thick Disks, or in their possible LMC counterparts. We discuss in detail how further observations of microlensing rates, and of the moments of the distribution of event durations, can help resolve the issue of the Brown-Dwarf location, and eventually provide information on the mass function of the dark objects.

On the mass of the dark compact halo objects

Recently the French EROS Collaboration and the American-Australian MACHO Collaboration have reported the observation of altogether three possible microlensing events by monitoring over several years the brightness of millions of stars in the Large Magellanic Cloud. For each of these events, assuming they are due to microlensing, we compute the most likely mass for the dark compact halo object, which acted as gravitational lens. The most likely masses are 0.12, 0.31 and 0.38 M⊙. The average mass calculated using the method of moments turns out to be 0.14 M⊙.

Extending the MACHO search to about 10 exp 6 solar masses

The search for a microlensing (changing light-curve) signature of massive compact halo objects (Machos) by the Macho Collaboration is currently believed to be sensitive in the range 10 -7 -10 2 M ⊙ . Microlensing events from higher mass objects last longer than the 4 yr duration of the planned experiments and therefore, according to current beliefs, cannot be distinguished from long-term variables. In fact the signature of Machos in the range 10 2 -10 3 M ⊙ can be distinguished from background events by the annual modulation in light magnification induced by the Earth's motion