Bibliography

Abazajian, K. N. et al. 2009, , 182, 543

Alard, C. 2000, , 144, 363

Alard, C. & Lupton, R. H. 1998, , 503, 325

Balog, Z. et al. 2013, Experimental Astronomy

Bionta, R. M., Blewitt, G., Bratton, C. B., Casper, D., & Ciocio, A. 1987, Physical Review Letters, 58, 1494

Chevalier, R. A. & Fransson, C. 1994, , 420, 268

Chugai, N. N. & Danziger, I. J. 2003, Astronomy Letters, 29, 649

Cox, P., Mezger, P. G., Sievers, A., Najarro, F., Bronfman, L., Kreysa, E., & Haslam, G. 1995, , 297, 168

Cutri, R. M. et al. 2003, 2MASS All Sky Catalog of point sources., ed. R. M. Cutri et al.

Dalcanton, J. J. et al. 2009, , 183, 67

Dale, D. A. et al. 2009, ApJ, 703, 517

Davidson, K. 1987, , 317, 760

de Jager, C. 1998, , 8, 145

de Jager, C. & Nieuwenhuijzen, H. 1997, , 290, L50

Dolphin, A. E. 2000, , 112, 1383

Draine, B. T. & Lee, H. M. 1984, , 285, 89

Egan, M. P., Clark, J. S., Mizuno, D. R., Carey, S. J., Steele, I. A., & Price, S. D. 2002, , 572, 288

Elitzur, M. & Ivezic, Z. 2001, , 327, 403

Fazio, G. G. et al. 2004, , 154, 10

Figer, D. F., McLean, I. S., & Morris, M. 1999, , 514, 202

Filippenko, A. V. 1997, , 35, 309

Fullerton, A. W., Massa, D. L., & Prinja, R. K. 2006, , 637, 1025

Gal-Yam, A. et al. 2007, , 656, 372

Gal-Yam, A., & Leonard, D. C. 2009, , 458, 865

Gardner, J. P. et al. 2006, , 123, 485

Gerke, J. R., Kochanek, C. S., Prieto, J. L., Stanek, K. Z., & Macri, L. M. 2011, , 743, 176

GSC2.2. 2001, VizieR Online Data Catalog, 1271, 0

Gvaramadze, V. V., Kniazev, A. Y., & Fabrika, S. 2010, , 405, 1047

Heger, A., Fryer, C. L., Woosley, S. E., Langer, N., & Hartmann, D. H. 2003, , 591, 288

Higgs, L. A., Wendker, H. J., & Landecker, T. L. 1994, , 291, 295

Horiuchi, S., Beacom, J. F., Kochanek, C. S., Prieto, J. L., Stanek, K. Z., & Thompson, T. A. 2011, , 738, 154

Hubble, E. & Sandage, A. 1953, , 118, 353

--. 1984, Science, 223, 243

Humphreys, R. M. & Davidson, K. 1994, PASP, 106, 1025

Humphreys, R. M., Jones, T. J., & Gehrz, R. D. 1987, , 94, 315

Humphreys, R. M., Davidson, K., & Smith, N. 1999, , 111, 1124

--. 2002, , 124, 1026

Humphreys, R. M. et al. 1997, , 114, 2778

Humphreys, R. M., Davidson, K., Jones, T. J., Pogge, R. W., Grammer, S. H., Prieto, J. L., & Pritchard, T. A. 2012, , 760, 93

--. 2006, AJ, 131, 2105

Ivezic, Z. & Elitzur, M. 1997, , 287, 799

Ivezic, Z., Nenkova, M., & Elitzur, M. 1999, ArXiv Astrophysics e-prints, 9910475

Jones, T. J. et al. 1993, , 411, 323

Kennicutt, Jr., R. C. et al. 2003, PASP, 115, 928

Khan, R., Stanek, K. Z., & Kochanek, C. S. 2013, , 767, 52

Khan, R., Stanek, K. Z., Kochanek, C. S., & Bonanos, A. Z. 2011, , 732, 43

Khan, R., Stanek, K. Z., Prieto, J. L., Kochanek, C. S., Thompson, T. A., & Beacom, J. F. 2010, ApJ, 715, 1094

Kennicutt, Jr., R. C. 1998, , 498, 541

Kochanek, C. S. 2011, , 743, 73

Kochanek, C. S., Szczygie, D. M., & Stanek, K. Z. 2012, , 758, 142

Kochanek, C. S. et al. 2008, , 684, 1336

Kourniotis, M. et al. 2014, , 562, A125

Kozlowski, S., Kochanek, C. S., Stern, D., Prieto, J. L., & Stanek, K. Z. 2010, Central Bureau Electronic Telegrams, 2392, 1

Liu, J. 2011, , 192, 10

Maeder, A. 1981, , 101, 385

Maeder, A. & Meynet, G. 1987, , 182, 243

--. 1988, , 76, 411

Massey, P., Olsen, K. A. G., Hodge, P. W., Strong, S. B., Jacoby, G. H., Schlingman, W., & Smith, R. C. 2006, AJ, 131, 2478

Mauerhan, J. C. et al. 2013, , 430, 1801

McQuinn, K. B. W. et al. 2007, ApJ, 664, 850

Meynet, G., Maeder, A., Schaller, G., Schaerer, D., & Charbonnel, C. 1994, , 103, 97

Mineo, S., Gilfanov, M., & Sunyaev, R. 2012, , 419, 2095

Moriya, T. J., Maeda, K., Taddia, F., Sollerman, J., Blinnikov, S. I., & Sorokina, E. I. 2014, , 439, 2917

Nieuwenhuijzen, H. & de Jager, C. 2000, , 353, 163

Ochsenbein, F., Bauer, P., & Marcout, J. 2000, , 143, 23

Ofek, E. O. et al. 2013, , 494, 65

--. 2014a, , 789, 104

--. 2014b, , 781, 42

Ott, C. D. 2009, Classical and Quantum Gravity, 26, 204015

Pastorello, A. et al. 2007, , 447, 829

--. 2013, , 767, 1

Poglitsch, A. et al. 2010, , 518, L2

Prieto, J. L., Brimacombe, J., Drake, A. J., & Howerton, S. 2013, , 763, L27

Remillard, R. A. & McClintock, J. E. 2006, , 44, 49

Rieke, G. H. et al. 2004, , 154, 25

Robinson, G., Hyland, A. R., & Thomas, J. A. 1973, , 161, 281

Schlegel, E. M. 1990, , 244, 269

Smartt, S. J. 2009, , 47, 63

Smith, N. 2009, ArXiv e-prints, 0906.2204

--. 2014, ArXiv e-prints

Smith, N. & McCray, R. 2007, , 671, L17

Smith, N. & Owocki, S. P. 2006, , 645, L45

Smith, N., Vink, J. S., & de Koter, A. 2004, , 615, 475

Smith, N. 2006, , 644, 1151

Smith, N. et al. 2008, , 686, 467

Smith, N., Li, W., Silverman, J. M., Ganeshalingam, M., & Filippenko, A. V. 2011, , 415, 773

Stanek, K. Z. et al. 2003, , 591, L17

Stoll, R. et al. 2011, , 730, 34

Stothers, R. B. & Chin, C.-W. 1996, , 468, 842

Szczygie, D. M., Gerke, J. R., Kochanek, C. S., & Stanek, K. Z. 2012, , 747, 23

Tiffany, C., Humphreys, R. M., Jones, T. J., & Davidson, K. 2010, , 140, 339

Ueta, T., Meixner, M., Dayal, A., Deutsch, L. K., Fazio, G. G., Hora, J. L., & Hoffmann, W. F. 2001, , 548, 1020

Vink, J. S., Davies, B., Harries, T. J., Oudmaijer, R. D., & Walborn, N. R. 2009, , 505, 743

Voors, R. H. M. et al. 2000, , 356, 501

Wachter, S., Mauerhan, J. C., Van Dyk, S. D., Hoard, D. W., Kafka, S., & Morris, P. W. 2010, , 139, 2330

Woods, P. M. et al. 2011, , 411, 1597

Wright, E. L. et al. 2010, , 140, 1868

Figure 1: The spectral energy distributions (SEDs) of the dust-obscured massive star $\eta$Car (dash-dot line, e.g., Robinson et al.1973; Humphreys & Davidson1994), ``ObjectX'' in M33 (dashed line; Khan et al.2011), and an obscured star in M81 that we identify in this paper (M81-12, solid line). All these stars have SEDs that are flat or rising in the Spitzer IRAC 3.6, 4.5, 5.8 and $8.0\,\micron$ bands (marked here by solid circles). The three shortest wavelength data-points of the M81-12 SED are from HST $BVI$ images. The $24\,\micron$ measurements of both ObjectX and M81-12 are from Spitzer MIPS while the dotted segments of their SEDs show the Herschel PACS 70, 100, and $160\,\micron$ upper limits.

Figure: The Hubble, Spitzer, and Herschel images of the region around M81-12. In the left panel, the radii of the circles are $0\farcs25$ (5 ACS pixels) and $1\farcs43$ (IRAC $4.5\,\micron$ PSF FWHM), and the source at the position of the smaller circle in the left panel is the brightest red point source on the CMD (Figure4, left panel). The red line in each panel is the size of a PACS pixel ($3\farcs2$).

Figure: The Spitzer MIPS 24, 70 and $160\,\micron$ (top row) and Herschel PACS 70, 100 and $160\,\micron$ (bottom row) images of the region around the object N7793-9. The higher resolution of the PACS images helps us set tighter limits on the far-IR emission from the candidates.

Figure 4: The $F606W$ ($V$) vs. $F435W-F606W$ ($B-V$) color magnitude diagram (CMD) for all HST point sources around M81-12. The three large solid triangles denote sources located with the $0\farcs3$ matching radius. The small open triangles show all other sources within a larger $2\farcs0$ radius to emphasize the absence of any other remarkable sources nearby. The circle marks the source at the position of the smaller circle in the left panel of Figure2, which is the brightest red point source on the CMD. The excellent ($<0\farcs1$) astrometric match and the prior that very red sources are rare confirms that this source is the optical counterpart of the mid-IR bright red Spitzer source.

Figure 5: The differential light curves of some of the candidates in M81 and NGC2403 obtained from the Large Binocular Telescope. The data spans the period from March2008 to January2013. The $U$ (squares), $B$ (triangles), $V$ (circles), $R$ (crosses) differential magnitudes are offset by $+0.3, +0.1, -0.1, -0.3$mag for clarity.

Figure 6: The best fit model (solid line) of the observed SED (squares and triangles, the latter show flux upper limits) of M81-12 and the SED of the underlying, unobscured star (dashed line), as compared to $\eta$Car (dotted line). The best fit is for a $L_*\simeq 10^{5.9}\,L_\odot$, $T_*\simeq 7900\,K$ star obscured by $\tau _V\simeq 8$, $T_d\simeq 530\,K$ silicate dust shell at $R_{in}=10^{16.1}$cm.

Figure 7: Same as Figure6, but showing all the obscured stars that we identified as compared to M33VarA, $IRC+10420$, and $\eta$Car. The solid line shows the best fit model of the observed SED, and the dashed line shows the SED of the underlying, unobscured star. M33VarA and $IRC+10420$ are shown on separate panel while $\eta$Car is shown on every panel (dotted line).

Figure 8: The SEDs of the 16 candidates that we concluded are not stars (points and solid lines) as compared to $\eta$Car (dotted line).

Figure: Luminosities of the obscured stars as a function of the estimated ejecta mass determined from the best fit model for each SED. The dashed lines enclose the luminosity range $\log(L/L_{sun})\simeq5.5-6.0$. We do not show N7793-3 for which we have no optical or near-IR data. $IRC+10420$ (square), M33VarA (triangle), and $\eta$Car (star symbol) are shown for comparison. The error bar corresponds to the typical $1\sigma$ uncertainties on $L_{bol}$ ($\pm 10\%$) and $M_e$ ($\pm 35 \%$) of the best SED fit models.

Figure 10: Same as Figure9, but for different dust types and temperature assumptions. The top row shows the best silicate (left), graphitic (center), and the better of the two (right, same as Figure9) models. The middle and bottom rows show the best fit models for graphitic and silicate dust at fixed stellar temperatures of 5000K, 7500K and 20000K. The only higher luminosity case in the fixed temperature model panels is N2403-4, for which the best fit models have significantly smaller $\chi ^2$ and lower luminosities for both dust types.

Figure 11: Cumulative histograms of the dust shell radius $R_{in}$ for the newly identified stars excluding N7793-3. The dotted lines, normalized to the point where $F(<R_{in})=0.5$, shows the distribution expected for shells in uniform expansion observed at a random time.

Figure 12: Elapsed time $t= R_{in} {v_{e100}^{\,\,-1}}$ as a function of the estimated ejecta mass $M_e$ for the best fit graphitic models. The mass and radius are scaled to $\kappa _V=100\,\kappa _{100}$cm$^2$gm$^{-1}$ and $v_e=100\,v_{e100}$kms$^{-1}$, and can be rescaled as $t\propto {v_e^{-1}}$ and $M_e\propto {\kappa _V^{-1}}$. The error bar shows the typical $1\sigma$ uncertainties on $t$ ($\pm 15\%$) and $M_e$ ($\pm 35 \%$) of the best SED fit models.. The three dotted lines correspond to optical depths $\tau _V=1,10$ and $100$. We should have trouble finding sources with $\tau _V<1$ due to lack of mid-IR emission and $\tau _V\gtrsim 100$ due to the dust photosphere being too cold (peak emission in far-IR). The large $t$ estimate for $\eta$Car when scaled by $v_{e100}$ is due to the anomalously large ejecta velocities ($\sim 600$kms$^{-1}$ along the long axis (Smith2006; Cox et al.1995) compared to typical LBV shells ($\sim 50$kms$^{-1}$, Tiffany et al.2010).

Table 1: PACS Aperture Definitions

Band ($\micron$)	Pixel Scale	$R_{ap}$	$R_{in}$	$R_{out}$	Ap. Corr.
$70\,\micron$	$3\farcs2$	$6\farcs4$	$60\farcs8$	$70\farcs4$	${0.72}^{-1}$
$100\,\micron$	$3\farcs2$	$6\farcs4$	$60\farcs8$	$70\farcs4$	${0.69}^{-1}$
$160\,\micron$	$6\farcs4$	$12\farcs8$	$121\farcs6$	$140\farcs8$	${0.78}^{-1}$