<<< Rubab, Home

Census of Self-Obscured Massive Stars in Nearby Galaxies with Spitzer:
Implications for Understanding the Progenitors of SN 2008S-Like Transients

Rubab Khan, K. Z. Stanek, J. L. Prieto, C. S. Kochanek, Todd A. Thompson, J. F. Beacom

Abstract:

A new link in the causal mapping between massive stars and potentially fatal explosive transients opened with the 2008  discovery of the dust-obscured progenitors of the luminous outbursts in NGC 6946 and NGC 300. Here we carry out a  systematic mid-IR photometric search for massive, luminous, self-obscured stars in four nearby galaxies: M33,   NGC 300, M81, and NGC 6946. For detection, we use only the 3.6 micron and 4.5 micron IRAC bands, as these can still  be used for multi-epoch Spitzer surveys of nearby galaxies (=<10 Mpc). We combine familiar PSF and  aperture-photometry with an innovative application of image subtraction to catalog the self-obscured massive stars in  these galaxies. In particular, we verify that stars analogous to the progenitors of the NGC 6946 (SN 2008S) and  NGC 300 transients are truly rare in all four galaxies: their number may be as low as ~1 per galaxy at any given moment.  This result empirically supports the idea that the dust-enshrouded phase is a very short-lived phenomenon in the lives of  many massive stars and that these objects constitute a natural extension of the AGB sequence. We also provide mid-IR  catalogs of sources in NGC 300, M81, and NGC 6946.

Pre-print draft: arxiv:1001.3681

Link to Journal: Astrophysical Journal 715 (2010) 1094-1108.

NGC 300 mid-IR Catalog: Table 6

M81 mid-IR Catalog: Table 8

NGC 6946 mid-IR Catalog: Table 10



M33_IRAC_4.5M33_differenced

Fig. 1.— M33 4.5 μm IRAC image (left) and [3.6]−[4.5] differenced image (right). The image covers an area of ~ 33' 33' (16001600 pixels, with 1.2"/pixel). This difference image is constructed by using image subtraction to scale and subtract the 3.6 μm image from the 4.5 μm image including the necessary corrections for the PSF differences. All the normal (non-red) stars “vanish” in the differenced image leaving the stars with significant dust emission.



NGC300_IRAC_4.5NGC300_differenced

Fig. 2.— NGC 300 4.5 μm IRAC image (left) and [3.6] − [4.5] differenced image (right), as in Figure 1. The image covers an area of ~ 15' 15' (1250 1250 pixels with 0.75"/pixel).



M81_IRAC_4.5M81_differenced

Fig. 3.— M81 4.5 μm IRAC image (left) and [3.6]−[4.5] differenced image (right), as in Figure 1. The image covers an area of ~ 18'18' (1450 1450 pixels with 0.75"/pixel). The saturated center of M81 has been masked for data reduction purposes.



NGC6946_IRAC_4.5NGC6946_differenced

Fig. 4.— NGC 6946 4.5 μm IRAC image (left) and [3.6] − [4.5] differenced image (right), as in Figure 1. The image covers an area of ~ 12' 12' (1000 1000 pixels with 0.75"/pixel). The 4.5 μm SINGS archival image contains many artifacts that significantly affect the subtracted image. For example, bright stars show bright “halos” in the 4.5 μm image that appear as rings in the subtracted image.



Class-A

Fig. 5.— A Class–A object in NGC 300 visible at both 3.6 μm (top left) and 4.5 μm (top center). The [3.6]−[4.5] difference (top right), 5.8 μm (bottom left ), and 8.0 μm (bottom center) images are also shown. Each panel is ~52.5" on its sides.



Class-B

Fig. 6.— A Class–B object in M33 visible only at 4.5 μm (top center) and not at 3.6 μm (top left ). The 3.6 μm magnitude is determined at the position of the 4.5 μm source. The [3.6] − [4.5] difference (top right), 5.8 μm (bottom left ), and 8.0 μm (bottom center) images are also shown. Each panel is ~106.4" on its sides.



Class-C

Fig. 7.— A Class–C object in M81 found only in the [3.6] − [4.5] wavelength differenced image (top right) but at neither 3.6 μm (top left) nor 4.5 μm (top center). The magnitudes are determined through aperture-photometry at the location identified in the differenced image. The 5.8 μm (bottom left) and 8.0 μm (bottom center) images are also shown. Each panel is ~60.0" on its sides.



NGC 300 Transient

Fig. 8.— The NGC 300 transient progenitor is identified as a Class–A object detected at both 3.6 μm (top left) and 4.5 μm (top center). The [3.6] − [4.5] difference (top right), 5.8 μm (bottom left), and 8.0 μm (bottom center) images are also shown. Each panel is ~60.0" on its sides.



SN 2008S

Fig. 9.— The SN 2008S progenitor, identified in NGC 6946 as a Class–B object, is detected only at 4.5 μm (top center) but not at 3.6 μm (top left ). The differenced image (top right) makes it clear that had we missed it at 4.5 μm, it would be detected without any confusion as a Class–C object. The 5.8 μm (bottom left ), and 8.0 μm (bottom center) images are also shown. Each panel is ~41.3" on its sides.


M33 CMDM33 SED

Fig. 10.— Mid-infrared color-magnitude diagram (left) and EAGB SEDs (right) for M33. The apparent magnitude at 4.5 μm is plotted versus [3.6]−[4.5] color for all sources detected in both 3.6 μm and 4.5 μm images through PSF-photometry (black dots). For comparison, the positions of the progenitors of NGC 300 (black circle) and SN 2008S (black square, lower limit in color) are also shown (Prieto et al. (2008); Prieto (2008)), and the 4.5 μm absolute magnitude scale is shown on the right. The [3.6]−[4.5] > 1.5 and M4.5 < −10 selection for extremely red and bright objects, following the criteria used by Thompson et al. (2009), is shown by the dashed lines. The EAGB candidates that meet these criteria are shown with different symbols sorted according to the stage of the search at which they were identified. The red circles and open red squares indicate Class–A objects identified through PSF and aperture-photometry, the blue triangles indicate Class–B objects, and the green squares indicate Class–C objects (none in this case). Where applicable, the lower limits in color are indicated with arrows. Stars for which only m3.6 upper limits could be determined are not shown in the SEDs panel. The SEDs of the SN 2008S (red) and NGC 300 (green) progenitors are also shown. The 5.8 μm and 8.0 μm fluxes were determined through aperture-photometry for the locations identified in the 4.5 μm image. Due to significant PAH emission in these two bands, we consider the aperture-photometry measurements in these bands less reliable.



NGC300 CMDNGC300 SED

Fig. 11.— Mid-infrared color-magnitude diagram (left) and EAGB SEDs (right) for NGC 300. Symbols and colors used here are the same as in Figure 10. The SEDs of some fainter sources show a sharp decline at 5.8 μm before rising again at 8.0 μm due to PAH dominated background contamination.



M81 CMDM81 SED

Fig. 12.— Mid-infrared color-magnitude diagram (left) and EAGB SEDs (right) for M81. Symbols and colors used here are the same as in Figure 10. The SEDs of some fainter sources show a sharp decline at 5.8 μm before rising again at 8.0 μm due to PAH dominated background contamination. Sources for which 5.8 μm and 8.0 μm measurements could not be obtained at all due to contamination, only the 3.6 μm and 4.5 μm measurements are shown on the SEDs. The dashed line indicates an object for which only the 5.8 μm measurement could not be obtained.



NGC6946 CMDNGC6946 SED

Fig. 13.— Mid-infrared color-magnitude diagram (left) and EAGB SEDs (right) for NGC 6946. Symbols and colors used here are the
same as in Figure 10. The SEDs of some fainter sources show a sharp decline at 5.8 μm before rising again at 8.0 μm due to PAH dominated
background contamination. Sources for which 5.8 μm and 8.0 μm measurements could not be obtained at all due to contamination, only
the 3.6 μm and 4.5 μm measurements are shown on the SEDs. The dashed line indicates an object for which only the 5.8 μm measurement
could not be obtained.




All CMD

Fig. 14.— Mid-infrared color-magnitude diagrams for the six galaxies. Symbols and colors used here are same as in Figure 10, and the AGB region is shown in red. The small number of bright objects in the SMC CMD is largely due to the SMC survey (Bolatto et al. 2007) covering only portions of the SMC. The AGB regions of the M81 and NGC 6946 CMDs contain significant extragalactic contamination.



Sky Maps

Fig. 15.— Distribution of AGB stars (red dots) and EAGB stars (black filled circles), and the two 2008 transient locations (starred blue symbols) in the galaxies. The image scales and directions are indicated in each panel, as well as the R25 ellipse (green). For M33, the R25 ellipse lies outside of our angular selection region. The empty region at the center of M81 is due to the image mask that we used for the brightest, nearly saturated, central region of the galaxy. However, the notable absence of any AGB and EAGB candidates towards the central region of NGC 6946 is not artificial, as discussed in Section 3. We estimate that for NGC 300, M81, and NGC 6946, 90%, 70%, and 50% of our angular selection region is inside the R25 ellipse (minus the M81 mask and the “empty” central region of NGC 6946). We use these sky area estimates when scaling the extragalactic contamination from SDWFS.




Tables 1-3



Table 4



Table 5



Table 6
Full Table 6



Table 7



Table 8

Full Table 8



Table 9



Table 10

Full Table 10



Table 11



Conclusions:

We carried out a systematic mid-IR photometric search for massive, luminous, self-obscured stars in four nearby galaxies combined with existing data for the LMC and SMC. We use a combination of conventional PSF and aperturephotometry techniques along with an innovative application of image subtraction. We investigate the population of SN 2008S-like transient event progenitor analogs in these 6 galaxies. We report catalogs of mid-IR sources in three new galaxies (NGC 300, M81, and NGC 6946) and candidate extreme AGB stars in all 6.

Using our methods, bright and red extreme AGB stars can be inventoried to D =<10 Mpc despite Spitzer’s relatively poor angular resolution. The biggest current problem is that the archival data is insufficiently deep: even with our innovative “band-subtraction” technique, we simply need more photons to detect these extremely red stars in the more distant galaxies. A future multi-epoch survey using (warm) Spitzer could identify all EAGB candidates in nearby galaxies, characterize their variability, and pin down the contribution of these partly obscured stars to galaxy spectral energy distributions as a function of wavelength.

Finally, we again emphasize the point made by Thompson et al. (2009). Stars analogous to the progenitors of the SN 2008S and the NGC 300 transients are truly rare in all galaxies. At any moment there appears to be only ~1 true analog, and up to ~10 given a more liberal selection criterion, per galaxy. While completeness problems due to the limited depth of the archival data make it impossible to give exact scalings, they represent roughly 2E−4 of the red super giant population, ~1E−2 of the AGB population, and appear at a rate of order 50 EAGB stars per unit star formation (solar mass per year) using the liberal criteria (and an order of magnitude fewer if we use the more conservative one). Clarifying these scalings with stellar mass, star formation rate and metallicity requires larger and deeper surveys of nearby galaxies than can be accomplished with warm Spitzer or eventually with JWST.




Acknowledgements:


We thank Szymon Kozlowski for helping us estimate extragalactic contamination using the SDWFS data and Janice Lee for helpful discussions. We extend our gratitude to the SINGS Legacy Survey and LVL Survey for making their data publicly available. This research has made use of NED, which is operated by the JPL and Caltech, under contract with NASA and the HEASARC Online Service, provided by NASA’s GSFC. RK and KZS are supported in part by NSF grant AST-0707982. JLP acknowledges support from NASA through Hubble Fellowship grant HF-51261.01-A awarded by the STScI, which is operated by AURA, Inc. for NASA, under contract NAS 5-26555. KZS, CSK and TAT are supported in part by NSF grant AST-0908816. TAT is supported in part by an Alfred P. Sloan Foundation Fellowship. JFB is supported by NSF CAREER grant PHY-0547102.


References



<<< Rubab, Home