BOMBOLO - A Multiband Near-UV/Optical Imager for SOAR

Dani Guzman, Rodolfo Angeloni and Thomas H. Puzia
Center for Astro-Engineering
Ponti ficia Universidad Catolica de Chile

 

Abstract

BOMBOLO is a new astronomical multi-passband instrument proposed for SOAR 4m telescope on Cerro Pachón. As the first Chilean instrument of its kind, it is a three-arm imager covering the near-UV and optical wavelengths. The three arms work simultaneously and independently, providing synchronized imaging capability for rapid astronomical events. BOMBOLO will be able to address largely unexplored events in the minute-to-second timescales, with the following leading science cases: 1) Simultaneous Multiband Flickering Studies of Accretion Phenomena; 2) Near UV/Optical Diagnostics of Stellar Evolutionary Phases; 3) Exoplanetary Transits and 4) Microlensing Follow-Up. BOMBOLO is an ideal instrument to respond to the timely research needs from our community.

 

BOMBOLO optical design consists of a wide field collimator feeding two dychroic at 390 and 550 nm. Each arm includes a filter wheel and a science CCD, imaging a 7 x 7 arcmin2 field of view. It will be installed at SOAR visitor port. BOMBOLO has been approved by the SOAR Board of Directors as a visitor instrument and, provided its successful commissioning and performance, might become available to the entire community. This proposal is the first of its kind from a Chilean institution and it represents a major leap forward for astronomical technology development in Chile.

 

Description

The concept of an imager able to perform fast and simultaneous multi-passband photometry was suggested as the single answer to a series of scientific questions emerging within our team. Currently, these needs of fast simultaneous near-UV to optical photometry cannot be completely fulfilled by the astronomical instrumentation available to the Chilean community. In this sense, it is an exemplary case of science exploration feeding technology development in Chile.

The scientific goals and the consequent technological requirements to be achieved in order to make such an idea materialise brought together a vibrant collaboration between the astronomers and engineers at PUC, and eventually resulted in the BOMBOLO project. Generally speaking, the final goal of the project is to build a three-arms, simultaneous fast-readout, near-UV/optical CCD optical imager for the SOAR 4m telescope, located on Cerro Pachón in the fourth region of Chile. On one hand, BOMBOLO will facilitate accomplishing the science goals that the astronomical community is currently pursuing, and on the other, it will significantly drive and enhance the expertise of our team in astronomical instrumentation development. In that sense, BOMBOLO will be reviewed following the formal international review phase procedures, where an invited panel of experts from SOAR Telescope consortium, the National Optical Astronomical Observatoy (NOAO) and other independent international experts will assess the progress of the project.

We envision BOMBOLO as a cost-efficient instrument when compared to similar concepts proposed for other telescopes, yet with rather ambitious, modular and expandable concept design able to address broad scientific goals. In the framework of a general effort to gain independence and strengthen the position of instrument design and development in Chile, we plan to offer BOMBOLO to the entire astronomical community, in particular if the SOAR Observatory promotes BOMBOLO to become a facility instrument, once we have tested its capabilities and proved its performance and scientific productivity.

BOMBOLO will address the following science cases:

1) Accretion Processes at all Spatial Scales

Simultaneous multi-passband studies of flickering phenomena as a result of accretion processes play an important role in astronomy at basically all spatial scales, from proto-planetary systems to binary stars, up to galaxy clusters.  The associated phenomenology shows a wide-range of time-dependent behaviour, whose systematic investigation has so far been predominantly driven by detector development and their capabilities — more than by the underlying science — with the result that the temporal domain below 1 minute remains virtually unexplored. Generally speaking, we know that accretion processes are the expression of turbulence at work: in this sense, rapid photometric variability (hereafter simply “flickering”) that arises from quite a heterogeneous group of astrophysical systems (e.g., cataclysmic variables, X-ray binaries, young stellar objects, proto-planetary disks, etc.) can be seen as the result of turbulence in the accretion stream and/or disk. Flickering is also a powerful diagnostic of nuclear burning processes, and the study of any coherent short periodicity in the light curve could bring valuable information on a range of poorly studied magneto-hydrodynamic (MHD) phenomena, e.g. by determining the dynamics of gas stream/magnetic field interactions and by measuring the shape and precession of warped accretion disks.

Even if flickering has no specific pattern, one expects the longer-lasting fluctuations to have larger amplitudes, simply because the turbulent motion in a larger eddy of the stream occurs on longer timescales, and thus emits more light. An instrument like BOMBOLO, able to take three simultaneous images at minute-to-second scale intervals over a moderately large field of view, would thus pave the way for a multi-band systematic study of rapid variability phenomena at smaller (milli-magnitude level) and faster (≤10 sec) scales, nowadays barely investigated. We emphasize that even if flickering studies on a few bright objects date back to 90′s (see, e.g. Bruch 1992 A &A, 266, 237), virtually no systematic study on the spectral energy distribution of these rapid photometric variabilities has been performed so far. Even today, when multi-band information has to be gathered, the only solution is to observe the same target at the same time with more telescopes, e.g., like Zamanov et al. 2010 (MNRAS, 404, 381) did by using up to 4(!) telescopes for monitoring the flickering of RS Oph at quiescence. When BOMBOLO will be mounted on the 4m-class SOAR telescope, it will allow astronomers to perform fast multi-band photometry on much larger samples by observing fainter targets, and reaching new classes of astrophysical objects (e.g., the majority of Super Soft X-ray Sources are dimmer than B=16 mag). For this science case, the actual instrument field of view becomes a key parameter. To perform accurate differential photometry it requires a sufficiently large field of view (FoV) to simultaneously image a sufficient number of bright comparison stars: in Fig. 1 we show that a FoV of 7 x 7 arcminutes2 represents the best compromise between an optimal pixel scale while stillguaranteeing a reasonably large number of comparison stars.

Fig. 1: Probability to find a R=13 mag comparison star in the instrumental Field of View (FoV), as a function of galactic latitude and increasing FoVs, up to 10 arcminutes. A 7 x 7 arcmin2 FoV is the best compromise between an optimal pixel scale and a relatively large FoV

2) Stellar Populations Diagnostics

The globular clusters (GCs) in the Local Group have been the cornerstone of stellar astrophysics and stellar population synthesis models for centuries. The stellar populations in these star clusters have been regarded as the best approximations to simple stellar populations, which are characterized by a collection of coeval stars with one well-defined chemical composition. Spectroscopic studies of the past decades revealed peu à peu that some star clusters consist of multiple stellar sub-populations with distinct light-element abundance patterns, while recent HST studies bolstered these early findings by discovering that most massive star clusters in the Local Group contain at least two chemically distinct stellar sub-populations. High spectral resolution spectroscopic studies of GC member stars show also characteristic element (anti)-correlations that appear to scale with some GC properties. One of the most exciting recent findings is that the CNO element variations appear to scale with the GC initial dynamical parameters, such as initial half-mass radius and initial total dynamical mass. Such correlations emerge from the initial physical conditions of the earliest epochs of star cluster formation. Accessing the relative CNO abundances in tens of thousands of member stars becomes accessible through a combination of medium and narrow-band photometry at near-UV to optical wavelengths, and BOMBOLO would provide the necessary tool to conduct highly-efficient observations of stellar populations in a large number of Local Group GCs and nearby dwarf galaxies. This will provide invaluable first chemo-dynamic constraints for high-resolution hydrodynamical formation models for some of the oldest stellar populations in the Universe.

 

3) Exoplanetary Transits

The discovery and characterization of exoplanets, i.e. planets orbiting stars other than the Sun, is one of the most exciting and fast moving areas of research in astronomy today. Among the many exoplanets being continuously discovered, the planets that eclipse their stars as they orbit are especially interesting, as the fortuitous geometry allows the measurement of physical properties that are generally not accessible for other systems (accurate planetary masses and radii, relative orientation of orbital and stellar spin axis, atmospheric emission/transmission spectra). One of the most exciting possibilities transiting exoplanets allow is the study of their atmospheres. BOMBOLO would allow us to measure planetary radii at different wavelengths and therefore place constraints on their atmospheric composition, in a similar way to transmission spectroscopy. The advantage of this method is the higher signal-to-noise ratio than ground-based spectroscopy can achieve, which can give constrains on phenomena such as Rayleigh scattering in exoplanet atmospheres.

 

 

4) Microlensing Events Follow-Up

A direct consequence of the General Theory of Relativity is the deflection of light due to massive objects. The case in which the deflecting mass is that of a star is called microlensing. Star-star microlensing produces a discernible magnification of the flux of a background star by a foreground star on timescales of days to months. The “magnification curve” shown by such a microlensing event is an exceptional tool to probe different properties of both the foreground lens and the background source. For example, in the source plane, the shape of the magnification peak and its wavelength dependence allows the study of atmospheres of very distant (bulge) stars. In the lens plane, the properties of this magnification curve can be used to find planets orbiting around the lensing stars. Microlensing is the only planet-finding method that allows identifying planets in very distant stars (several kilo parsecs away). Furthermore, of all methods that use ground-based observations, microlensing has the greatest sensitivity toward finding earth-mass extrasolar planets. Several campaigns are constantly monitoring the galactic bulge for microlensing events (e.g. OGLE, MOA). When an interesting event is detected by such campaigns, an alert is issued triggering the observation of the event by several follow-up campaigns around the globe (e.g. MICROfun, PLANET, MiNDSTEp, etc). For such microlensing events, multi-band, high-cadence follow-up observations as facilitated by the BOMBOLO concept design are crucial ingredients in order to identify and accurately characterize planetary systems and the parent-star stellar atmospheres

For successfully addressing the above-mentioned science topics, we have gone through a technical feasibility study, which requires committing to the following engineering goals:

1) Instrument Requirements

Implement an optical design that will comply with the requirements found in Table 1:

Table 1: BOMBOLO instrument requirements

2) Optical Design

The optical design is geared towards optimizing the following sections:

a) A collimator

b) Dychroic mirrors close to a pupil plane

c) Cameras optimized for each band

Of these goals, the collimator is the hardest one to achieve. Fig. 2 shows a preliminary solution we have devised for this component, while using a paraxial solution for the camera at this stage, given that it is relatively easy to implement, due to the restricted passband:

Fig. 2: BOMBOLO Optical Design Concept

 

3) Detector System

Design and build a multi detector readout system that will allow multi-passband, synchronized observations with BOMBOLO. The specifications of the detectors sub-system are presented in Table 2.

Table 2: BOMBOLO detector requirements

 

4) Mechanical Enclosure: design and build a light-tight enclosure that allows the collimator, three arms and detector systems to be held aligned at all telescope angles.

 

5) Software: design and implement a software system for image acquisition and instrument control that will be compatible with SOAR observatory software systems. This software should incorporate a pipeline for reduction and archiving.

 

Table 3: Relevant Individuals

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