HST observations of a new V838 Mon type eruptive variable: IRAS

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1 Hubble Space Telescope <ID> Cycle 16 GO Proposal HST observations of a new V838 Mon type eruptive variable: IRAS Principal Investigator: Dr. Armin Rest Institution: National Optical Astronomy Observatories - CTIO Electronic Mail: arest@noao.edu Scientific Category: ISM AND CIRCUMSTELLAR MATTER Scientific Keywords: NOVAE, WINDS/OUTFLOWS/MASS-LOSS, VARIABLE AND PULSATING STARS, MAGELLANIC CLOUDS Instruments: WFPC2 Proprietary Period: 12 Orbit Request Prime Parallel Cycle Abstract In 2002, the variable V838 Mon went through an extraordinary and unique outburst. The outburst was large amplitude (delta(v)~9) and very luminous during which its spectrum remained that of an extremely cool supergiant. A rapidly evolving set of light echoes around V838 Mon was discovered soon after the outburst, and quickly became the most spectacular display of scattered light echoes ever seen. These light echoes were subsequently used to study the stellar physics of such an outburst, study the interstellar dust structure, and determine the distance to the AGB star through two independent direct geometric techniques. From SuperMACHO data, we have found a similar AGB variable star with an associated light echo complex in the LMC: IRAS The source presently has an apparent magnitude of R=13.5 and an absolute magnitude of at least M_R=-5. We are requesting HST time to get high resolution images of the echoes, which can be used to study the ISM, measure independent geometric distances to the LMC, and explore a rare but important phase in stellar evolution.

2 Dr. Armin Rest HST observations of a new V838 Mon type eruptive variable: IRAS Investigators: Investigator Institution Country PI Dr. Armin Rest National Optical Astronomy Observatories - CTIO Chile CoI# Dr. Chris Stubbs Harvard University USA/MA CoI Dr. Nicholas B. Suntzeff Texas A & M Research Foundation USA/TX CoI Dr. Chris Smith National Optical Astronomy Observatories - CTIO Chile CoI Dr. Knut A.G. Olsen National Optical Astronomy Observatories - CTIO Chile CoI Mr. Guillermo Damke National Optical Astronomy Observatories - CTIO Chile CoI Mr. Andrew Newman Washington University in St. Louis USA/MO CoI Ms. Arti Garg Harvard University USA/MA CoI Mr. Peter Challis Harvard University USA/MA CoI Dr. Douglas L. Welch McMaster University Canada CoI Mr. Jose Luis Prieto The Ohio State University Research Foundation USA/OH CoI Dr. Andy Becker University of Washington USA/WA CoI Mr. Gajus Miknaitis University of Washington USA/WA CoI Dr. Kem Cook Lawrence Livermore National Laboratory USA/CA CoI Dr. Mark Huber Lawrence Livermore National Laboratory USA/CA CoI Dr. Sergei Nikolaev Lawrence Livermore National Laboratory USA/CA CoI* Mr. Lorenzo Morelli European Southern Observatory - Chile Chile CoI* Dr. Dante Minniti Universidad Catolica de Chile Chile CoI Dr. Alejandro Clocchiatti Universidad Catolica de Chile Chile Number of investigators: 19 * ESA investigators: 2 # Admin CoI: Dr. Chris Stubbs Target Summary: Target RA Dec Magnitude IRAS V = 13.5 Observing Summary:

3 Dr. Armin Rest HST observations of a new V838 Mon type eruptive variable: IRAS Target Config Mode and Spectral Elements Flags Orbits IRAS WFPC2 Imaging F450W 2 WFPC2 Imaging F606W WFPC2 Imaging F814W WFPC2 Imaging F656N IRAS WFPC2 Imaging Polarimetry F606W 4 IRAS WFPC2 Imaging F450W 2 WFPC2 Imaging F606W WFPC2 Imaging F814W WFPC2 Imaging F656N IRAS WFPC2 Imaging Polarimetry F606W 4 Total prime orbits: 12

4 Scientific Justification V838 Monocerotis went through a spectacular explosion in 2002, an outburst so energetic that it became one of the brightest stars in the Local Group for a few weeks in 2002 at M V = 10, gaining 9 magnitudes in the V band from its typical quiescent brightness. It ejected so much debris that the material is still not optically thin. V838 Mon captured the attention of the world with its beautiful scattered light echoes observed by HST and was even followed by amateur astronomers. HST continues to observe the evolution of the echo complex with observations starting in Oct A regular cadence of images will allow astronomers to study the 3-D structure of the ISM as the light sweeps out into the dust in front of the star. The echoes around this star were subsequently used to study the stellar physics of this rare outburst (Kipper et al. 2004), test the Galactic reddening law (Munari et al. 2005), study the interstellar dust properties (Crause et al. 2003), construct 3-D maps of the circumstellar dust (Tylenda 2004, Tylenda et al. 2005), and determine the distance to V838 Mon though two independent direct geometric techniques using polarimetry and angular expansion rates (Bond et al. 2003, Tylenda 2004, Munari et al. 2005, Crause et al. 2005). In addition, the nature of the echoes around this variable has created a lively debate in the literature as to whether the dust material is simply ISM or whether it was ejected by the star. Thirty five refereed papers have been published on V838 Mon since 2002, and a conference on just this object was held at La Palma in May In the SuperMACHO project, we have imaged the bar of the LMC repeatedly and used an automated pipeline to subtract point-spread function matched template images from the recent epoch images. The resulting difference images are remarkably clean of the constant stellar background and are ideal for searching for variable objects. Using these difference images, we have discovered three new scattered light echoes associated with ancient supernovae (Rest, et al. 2005) and since then we have found a fourth. In addition, we have found a star, IRAS , and a light echo complex centered on it, remarkably similar to V838 Mon. Figure 1 shows an example of an image from October 2003 to the left (epoch 1, the template image), an image from December 2005 in the middle (epoch 2), and the resulting difference image to the right. In these images, white represents flux enhancements in the second epoch image and black in the first epoch image. The light echo rings are clearly visible, and extend out to 20 or 5 pc. Figure 2 shows the time series of difference images starting from 2001 to For all these difference images, the template used was the same, the image taken at December The star IRAS is a very luminous star (see Figure 3) listed in the catalog of obscured AGB stars in the LMC (Loup, et al. 1997). It is very bright in the near-ir, with 2MASS (epoch of observation 2000) and DENIS magnitudes of (I,J,H,K = 10.5, 8.3, 7.4, 6.8). It was detected by MSX at m(8.3µ) = 5.1 and also in the X-ray region by ROSAT. The brightness of the star in 1980 from the USNOA2.0 catalog is (B,R=21.2, 14.8). The star currently has a magnitude of R=13.5 in our epoch 2005 data. A guess would be that has a quiescent magnitude of about R=15, with a unknown amount of obscuration. 1

5 We are presently searching various archives for historical data of this star from which we can create a light curve. If the star did go into an AGB outburst, it may have reached V=8 for a few weeks if it was similar to V838 Mon. It would have stayed in outburst for about 100 days. Since in the last decades there have been several microlensing searches toward the LMC, the light curve of the star for this time period is very well known (see for example the MACHO light curve in Figure 4). Our incomplete time coverage did not detect an outburst of this size. Unfortunately, the OGLE data of this star is saturated. Most of our SuperMACHO data is also saturated, but since the CCD saturation is full-well spillage and not digital clipping, we will be able to recover the magnitude using aperture photometry. The SuperMACHO data, however, only cover about four months per year from September to December in the years We are exploring other sources or historical LMC photometric data, such as the patrol imaging of the LMC by W. Liller in Chile. As with V838 Mon, HST images of this object offer a rare opportunity to investigate: The 3-D structure of the ISM. If a light echo is observed at a time t after the eruption at an angular distance α to the source, then the line-of-sight distance z can be calculated as z = (ct)2 x 2 (1) 2ct where x = Dα is the coordinate in the plane of the sky (Couderc 1939; Chevalier 86), and D is the distance from the observer to the source star. Thus the 3-D location of the reflecting dust can be traced with multiple epochs. Even if the distance D is not known to high accuracy, the relative structure of the dust can still be determined. In this case, the reflecting dust is most likely a circumstellar dust shell as indicated by Spitzer images (see Figure 5): The top right panel shows that there is an infrared source at the position of IRAS at 24 µ. The SuperMACHO project has images at regular intervals in the last 6 years. This will allow us to map out the structures at the arcsecond scale. However, only HST is able to reveal the wealth of sub-arcsecond structure in these light echoes. It will allow us to measure much more precise proper motions and angular distances, complementing the ground based data. For example, Bond et al found a double helix feature in the HST images pointing back to V838 Mon. The result will be a clearer picture of the geometry of the ejection nebula from a massive star, thus enabling the fundamental questions of ejection mechanisms to be addressed. Distance to the LMC. Distance is one of the fundamental measurements in astronomy, since knowing the distance allows to determine the absolute luminosity and size of astronomical objects. However, historically it is very difficult to measure distances, and most distances are determined relying on the distance ladder, which uses a variety of distance indicators requiring calibrations and detailed theoretical modelling. Only in very rare causes is it possible to obtain geometrical distances by direct means, circumventing all the systematic biases introduced by the relative methods used to establish the distance ladder. Light echoes are one of the methods that allow such a 2

6 geometric distance determination under the right circumstances. One example is V838 Mon, to which the distance is determined by using ACS polarization measurements to an unprecedented accuracy of a couple of percent (Sparks 2005). The polarization images show a clear ring of maximum polarization. The accuracy of the distance measurements is mainly constrained by how accurate the angle of highest polarization can be determined by theoretical considerations (This angle is close to 90 o, but not exactly). Polarization images of IRAS provide a similar opportunity to determine the geometrical distance to the LMC. The distance to the LMC is one of the cornerstones of the extragalactic distance ladder, and there is still significant effort going on to improve its distance determination and decrease the systematic errors (e.g. Gieren et al. 2006, Keller et al. 2006). Cepheids, for example, are one of the most used distance indicators: with modern telescopes and detectors, light curves of individual Cepheid variables can be measured with good accuracy out to distances of about 20 Mpc (Freedman et al. 2001). However, it has been difficult to calibrate the Cepheid PL relation in the Milky Way, thus the Cepheids in the LMC have also been used to calibrate the fiducial PL relation. It is still unclear how much metallicity and other factors impact the PL relation and introduce systematic errors, e.g. Udalski et al. (1999) finds that the slope of the PL relation is different in the LMC compared to the MWG. A direct geometric measurement of the distance to the LMC is thus very important to improve/verify this anchor-point of the distance ladder. REFERENCES Bond et al. 2003, Nature, 422, 405 Couderc 1939, Annales d Astrophysique, 2, 271 Chevalier 1986, Apj, 308, 225 Crause et al. 2003, MNRAS, 341, 785 Crause et al. 2005, MNRAS, 358, 1352 Freedman et al. 2001, ApJ, 553, 47 Gieren et al. 2006, Mem. S. A. It., 77, 198 Keller et al. 2006, ApJ, 841, 642 Kipper et al. 2004, A&A, 416, 1107 Loup C., et al. 1997, Astron. Astroph. Sup. Ser. 125, 419 (1997) Munari et al. 2002, A&A, 389, L51 Munari et al. 2005, A&A, 434, 1107 Rest, et al. 2005, Nature, 438, 1132 Sparks 2005, ASP conf. series, 343, 452 Tylenda 2004, A&A, 414, 223 Tylenda et al. 2005, A&A, 441, 1009 Udalski at al. 1999, Acta Astronomica, 49, 201 Zaritsky et al. 2002, AJ, 123, 855 3

7 Figure 1: CTIO 4m images showing the difference of IRAS between 20 October 2003 (left panel) to 29 December 2005 (middle panel). The scale of each image is 20 arcseconds. By careful subtraction (right panel) the light echoes becomes clear: white represents flux enhancements in the 29 December 2005 image and black in the 20 October 2003 image. Description of the Observations The observing plan for IRAS follows the previous HST observations of V838 Mon and Eta Carinae. We are requesting 2 epochs of observations in cycle 16. First epoch will use WFPC2 with F450W, F606W, F656N and F814W for 2 orbits to make a spectacular sub-arcsecond image of the light echoes. If awarded, we would work with the HST Heritage Group to make the image available to the public. The combination of the exposures should be able to provide the needed data to remove the contamination from the PSF of the bright central star. To make the polarization measurements of the light echoes we plan to use the POLQ with F606W. The F606W filter provides the highest system throughput. The observations will be done by placing the target, which is about 20 in size, in each of the 4 CCD s to obtain the polarization the 4 different orientations 0, 45, 90 and 135 degrees. In one orbit, APT tells us 1 exposure of 60 seconds and 4 at 300 seconds are available for IRAS in the LMC. By using a number of exposures we can drizzle the exposures to help improve the resolution and overall image quality. The light echoes are about 22 mag/arcsec 2, the WFPC ETC output indicates a s/n of 10 per pixel will be obtained with 1200 seconds. This s/n is sufficient since the light echoes are extended and we can average over lots of pixels with the same distance to the source star. We request 4 orbits to obtain the polarization with F606W. For the second epoch, we will re-image with the same plan six months later. It is important to have 2 epochs since this field in the LMC is very crowded, therefore the light echoes are best extracted with the difference imaging technique. Special Requirements If awarded, we request the first observation to be early in Cycle 16. 4

8 Figure 2: Difference images of IRAS from the SuperMACHO program from November 2001 to December Rows 1-5 show the difference images taken in [northern] fall 2001, fall 2002, fall 2003, fall 2004, and fall 2005, respectively. An image from 20 October 2003 is used as the template. In these images, white represents flux enhancements in the labeled epoch while black represents flux enhancements in the template epoch. Coordinated Observations Justify Duplications There is archival imaging data close to the position, but the star IRAS is off the field. Program GO (same group of investigators) has 1 orbit scheduled in spring 2007 to make a reconnaissance of this interesting target. 5

9 ! #"%$'&( )+*-,/.! :24<; Figure 3: color-color diagram. The field stars are from the MCPS catalog (Zaritsky et al. 2002) Figure 4: The light curve of IRAS from the MACHO program from

10 Figure 5: Top left: SuperMACHO difference image (VR filter). Top right: Spitzer 24 µ image. Bottom left: Spitzer 70 µ image. Bottom right: Spitzer 160 µ image. The green circle has a radius of 25 and is centered on IRAS