V900 MON AND THOMMES' NEBULA: A NEW FUor IN MONOCEROS

Figure 1 shows the region around V900 Mon on the first-epoch (epoch 1953.1) red Palomar Sky Survey and as seen on the discovery image (RGB+Hα, epoch 2009) by Jim Thommes. Only a barely detectable, slightly extended source is visible in the early image, whereas a bright nebulosity has emerged in the 2009 image. The Two Micron All Sky Survey (2MASS) catalog lists a bright near-infrared source (2MASS 06572222-0823176) within less than an arcsecond from the very faint object visible on the red POSS1 plate. We conclude that the partly embedded star has undergone a major increase in brightness between 1953 and 2009.

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Although located in Monoceros, V900 Mon is not directly associated with the prominent Mon R2 star-forming region, but is located further to the southeast at Galactic coordinates l = 221.07 and b = −2.51; this places it toward the small L1656 cloud, which is part of a long filamentary cloud complex, studied in CO by Maddalena et al. (1986) and Wilson et al. (2005) and in ¹³CO by Kim et al. (2004). They suggested that the filament forms a bridge between the Mon R2 complex and the CMa OB1 clouds. The stars of the Mon R2 complex were studied by Herbst & Racine (1976), who determined a distance of 830 ± 50 pc. Lombardi et al. (2011) suggested a distance of 905 ± 37 pc based on infrared star counts. The distance to the CMa OB1 complex has been estimated in several studies, and the resulting values cluster around 1000–1150 pc (see Gregorio-Hetem 2008); Kaltcheva & Hilditch (2000) have done uvbyβ photometry of OB stars in the region and find a distance of 990 ± 50 pc, while Lombardi et al. (2011) find a distance of 1150 ± 64 pc. In summary, we adopt a distance of 1100 pc for V900 Mon.

Several compact regions of star formation are located north of V900 Mon. A small nebulous clustering of stars, RNO 78 (Cohen 1980), is found about 4 arcmin north of V900 Mon, and has been studied by Bica et al. (2003, their object no. 93), who adopt a distance of 1100 pc.

About 12 arcmin NNW of V900 Mon is the more extensive Lynds Bright Nebula 1022, which is a small, largely unstudied, H ii region cataloged as [KC97c]G220.9–02.5 by Kuchar & Clark (1997).

We have searched the L1656 cloud around V900 Mon for Hα emission line stars, and found three to the northwest, in the general vicinity of RNO 78, the little nebulous group of stars associated with the source IRAS 06548–0815 (J2000 coordinates):

L1656-Hα1   6:57:05.6 –08:22:02

L1656-Hα2   6:57:14.5 –08:19:55

L1656-Hα3   6:57:14.9 –08:19:17.

Figure 2(a) shows an r'-band image of V900 Mon taken at the Gemini-North telescope. A bright, compact reflection nebula ∼25'' long opens up to the SW, and fainter filaments extend almost an arcminute from the source. The innermost contours are not circular, suggesting that in the optical we do not directly see the star, but mainly a very compact reflection nebula. Figure 2(b) shows a similar J-band image, and here the innermost contours are circular with an FWHM of 08, compared to 07 for nearby stars, suggesting that in the near-infrared the star itself may be visible.

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Table 1 lists g'r'i'z' photometry in a 1'' aperture (with seeing of 05). In a 5'' aperture, the brightness increases significantly: g' = 16.20, r' = 14.55, i' = 13.35, and z' = 12.50. This shows that, at least in the optical, photometry is strongly dependent on aperture.

JHK'L' photometry obtained on 2010 January 2 is listed in Table 1, revealing a bright near-infrared source. Additional narrowband images in H₂ and [Fe ii] show no evidence for shocked outflow near the star. Also, we have obtained laser-guided AO K-band imaging with NIRC2 on the Keck II telescope, and find no evidence for any companion at distances larger than ∼01.

V900 Mon was detected in the all-sky surveys of 2MASS, Midcourse Space Experiment (MSX), WISE, and AKARI, and the resulting photometry is listed in Table 1 and discussed in Section 4.

The GMOS optical low-resolution spectrum of V900 Mon reveals a red continuum dominated by absorption lines, with no emission present. The only strong lines are Hα and the far-red Ca ii triplet. The clear absence of any TiO bands indicates that the spectrum is earlier than mid- to late-K. Little other information can be gleaned from the spectrum.

A high-resolution spectrum of V900 Mon, as described in Section 2, has provided further insights into the nature of the star. Figure 3 shows three prominent lines from this spectrum. The Na i D_{1, 2} doublet at λλ 5890/5896 display the prominent blueshifted wings that characterize the massive cool winds found in FUors. The spectrum near the Na i D_{1, 2} doublet is weak in our exposure due to the considerable reddening of the star, so we have binned the data in this wavelength range by a factor of five. The λ5890 line has wings reaching a velocity of ∼140 km s⁻¹, whereas the λ5896 wing is measured to ∼125 km s⁻¹. Absorption features are seen in the blue wings of both the D₁ and D₂ lines at v_hel = +8, −40, and −80 km s⁻¹; the deepest point in both lines is at +25 km s⁻¹ and is probably interstellar. Due to the high noise in this part of the spectrum, these values are lower limits to the width of these absorption troughs. Within the noise level, no evidence for emission is seen.

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The Hα emission line shows a classical P Cygni profile, with a sharp, narrow emission peak (v_hel = +48 km s⁻¹) and a massive blueshifted trough with a width of 400 km s⁻¹. Structure is seen at the bottom of the trough, with minima at −150 and −9 km s⁻¹ (v_hel). Further to the red, two of the Ca ii triplet lines at λλ 8498 and 8662 are prominent; the line at λ8542 fell between orders in the spectrum. Figure 3 also shows the λ8662 line. It displays a prominent P Cygni profile, but because of overlap with the broad Paschen line P13 at λ8665, it is not possible to measure the extent of the blueshifted trough. The deep minimum is at (v_hel) +12 km s⁻¹ and the shallower minimum is at −34 km s⁻¹, and the emission peak is at +42 km s⁻¹.

Despite the somewhat underexposed spectrum, Li i λ6707 is clearly detected (EW 0.027 A), confirming the youth of the star. Also the blend centered on λ6497 and dominated by Ba ii is evident, a feature that is prominent in FUors and other very low gravity stars.

The infrared low-resolution spectrum from 1.2 to 2.4 μm (Figure 4(a)) shows prominent CO bandhead absorption as well as major depressions in the continuum due to water vapor, clearly indicating a very late-type spectrum, in marked contrast to the optical spectrum. The appearance is strikingly similar to that of FU Ori itself, except that V900 Mon is much more reddened. Given the similarity of the two objects, we have dereddened the spectrum of V900 Mon until the overall slope matches that of FU Ori (Figure 4(a)). The best fit is achieved for an A_V of 13 mag. Given that FU Ori has only small reddening, this is therefore a good estimate of the extinction toward V900 Mon.

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We have obtained optical and near-infrared photometry of V900 Mon. In order to understand any possible evolution in the light curve of the star, we have compiled all photometry known to us in Table 1. V900 Mon is too faint (>20 mag) on the red 1953 POSS plate to yield a meaningful magnitude, but two red plates from 1971 and 1975 show clearly an 18th magnitude object, suggesting that brightening had commenced by then. The near-infrared data from DENIS and 2MASS, both from 1998, reveal a bright K-band source, which had further brightened to K = 7.3 when we observed it in early 2010. It is noteworthy that the DENIS K magnitude is 0.3 mag brighter than the 2MASS K magnitude obtained eight months later, and so, if real, the overall brightness increase does not seem to be strictly monotonic. Our optical photometry in late 2010 reveal an object with r = 16.2, suggesting an optical brightening between 1953 and 2010 of 4 mag or more in this time interval. In the decade between 1996/7 and 2006, V900 Mon brightened ∼1.5 mag at 8 μm as detected by MSX and Spitzer. These limited data points do not allow a clear description of the rise of V900 Mon, but are more consistent with a slow rise time akin to that of V1515 Cyg than the sudden rise of FU Ori (Herbig 1977).

In addition to the optical and near-infrared photometry discussed above, we have used the WISE and AKARI mid- and far-infrared photometry, obtained in 2010 and 2006, respectively, to plot the observed energy distribution (open circles) of V900 Mon in Figure 4(b). We note that the photometric data points employed are non-simultaneous, obtained between 2006 and 2010, so it is possibly affected by variability. Adopting the A_V ∼ 13 mag extinction derived from the near-infrared spectrum, we have de-reddened the energy distribution (crosses) in Figure 4(b). The result shows a remarkable similarity to FU Ori itself (black dots), with the principal exception that V900 Mon has a larger mid-infrared excess. The flat energy distribution suggests a large range in temperatures of the circumstellar material.

Using optical g'r'i'z' photometry, near-infrared 2MASS JHK photometry, WISE mid-infrared photometry, and AKARI mid- and far-infrared photometry, we have measured the luminosity of V900 Mon as 106 L_☉ at the assumed distance of 1100 pc. This high luminosity is consistent with the luminosity range found for other FUors; in the same wavelength range as above, FU Ori has a luminosity of 164 L_☉ at an assumed distance of 450 pc.

The data presented in the preceding section reveal an object that shows

• 1.  
  a major brightening sometime in the last half century, possibly still ongoing;
• 2.  
  P Cygni profiles with massive blueshifted troughs at prominent optical lines;
• 3.  
  a spectrum that is increasingly late with increasing wavelength;
• 4.  
  deep infrared CO bandhead absorption;
• 5.  
  a reflection nebula, and lithium in the spectrum.

These are all characteristics which—taken together—uniquely identify V900 Mon as a new member of the rare FU Orionis class. V900 Mon is remarkably similar to the prototype object FU Orionis, except that it is much more embedded, and in this respect is more similar to the FUor V733 Cep (Reipurth et al. 2007). The Spitzer photometry places V900 Mon among the Class I sources, bordering the Class II sources; see Allen et al. (2004).