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We made a preliminary selection of candidate members using ancillary optical photometric data, to reduce the number of false positives from variable field sources. Of the recent optical surveys that covered our field, the most complete was the INT Photometric H-alpha Survey (IPHAS). Since the IPHAS initial data release had been withdrawn from publication at the time of writing, we could only use IPHAS data that had been collected byRebull et al. for sources with Spitzer counterparts. Nonetheless, we had bothr andiphotometry for 83% of our sample.

InFigure 6.2we show the colors of our variable sources in the IPHAS bands. The area between the two red lines is where main sequence stars are expected to lie (Drew et al.,2005). Most sources fall in this region, indicating they have at most modest Hαexcesses. Sources above the region are emission-line stars, confirmed by spectra where we have them. We note that emission-line stars selected byWitham et al.(2008) were already included in the member list compiled byRebull et al. (2011). We do not include them in the sample for variability statistics in subsection 6.5.1 and section 6.6, unless they were independently selected on the basis of variability.

The 128 sources below the main sequence locus appear to have poor photometry in either the IPHAS r or Hα bands; spectra show a mix of emission-line and nonemitting objects. Given the likelihood that the measurements are misleading, we do not exclude sources with lowr−Hαon the basis of their colors.

In Figure 6.3we show a color-magnitude diagram of our sources. Sources are scattered over a broad swath of color-magnitude space. The empty region in the lower right is caused by our selection ofRPTF≤20 sources. InFigure 6.3bwe show only the variables whose spectra were examined for youth indicators. Nearly all the high-confidence members have r−i <2; among r−i > 3 most sources are likely non-members (specifically, giants), although some have Balmer emission.

We use Figure 6.3to reject sources that are too faint to be main-sequence or pre-main-sequence stars associated with the North America Nebula complex. The figure shows both isochrones and the main sequence at a distance of 600 pc; we adopt this distance rather than the 520 pc we assume

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Figure 6.2: Anr−Hα vs. r−i color-color plot. The solid red lines show the upper and lower boundaries of the region where main-sequence stars can be found, if the extinction to individual sources is allowed to take on arbitrary values (Drew et al.,2005). The black arrow shows the (approximate) extinction vector fromCardelli et al.(1989), assuming

(a)All variables (b)Spectroscopic targets only

Figure 6.3: Anivs. r−icolor-magnitude plot. The red curves show, from top to bottom, synthetic photometry ofD’Antona & Mazzitelli(1997) isochrones at 1 Myr, 3 Myr, and 10 Myr at a distance of 600 pc. The blue curve is the empirical main sequence fromKraus & Hillenbrand(2007) at the same distance. The black arrow shows the extinction vector fromCardelli et al.(1989), assumingRV = 3.1. Extinction tends to shift sources onto

younger isochrones. In the right panel, red triangles are high-confidence members, blue squares low-confidence members, and black squares likely non-members, as defined in subsection 6.4.2. Very red sourcesr−i_&1 tend to be much more luminous than other sources in the field, indicating that they are background giants.

throughout the rest of this chapter to allow for a more conservative rejection of background sources. The reddening vector tends to raise sources above the main sequence, so any sources below it should be non-members with high probability, regardless of their extinction.

Since many IPHAS sources in the region have i photometry but not r, and cannot be placed on the color-magnitude diagram directly, we also reject sources with i > 18.7. Sources that have

i >18.7 but fall within our detection limit of RPTF ≤20 are assumed too blue to fall above the main sequence, although we caution that the IPHAS i photometry is unlikely to have been taken while the source was near its median PTF magnitude.

There are five sources that are well below the main sequence line, yet are high-confidence spec- troscopic members. These sources are:

FHO 89 (i= 18.0, r−i= 1.41) The IPHASr magnitude of 19.4 is considerably fainter than the observed rangeRPTF ∼17.7-18.4. However, this source is clearly blended in the PTF image, so likely neither the IPHAS nor PTF photometry are reliable.

FHO 299 (i = 18.4, r−i = 1.53) is a source with a 3-magnitude peak-to-peak amplitude that appears to have been observed by IPHAS close to minimum light. The source would spend most of its time above our background source cutoff.

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FHO 558 (i= 13.7, r−i= 0.23) is V751 Cyg, a known cataclysmic variable. This source illustrates that even high-confidence members are not necessarily young stars.

FHO 568 (i= 18.5, r−i= 1.48) The IPHASrmagnitude of 20.0 is considerably fainter than the observed rangeRPTF∼17.3-18.7. However, this source is a close double in SDSS images, so likely neither the IPHAS nor PTF photometry are reliable.

FHO 1891 (i = 17.0, r −i = 0.93) is a young star with both a strong emission-line spectrum and a strong infrared excess. Variability cannot account for its position in the IPHAS color- magnitude diagram.

Of the five sources, then, only two, FHO 299 and FHO 1891, represent high-quality detections of young stars. This supports the idea that most of the sources below the blue line in Figure 6.3 are likely non-members. Therefore, we can remove sources below the main sequence line from our sample without significantly sacrificing completeness.

Of 2,705 variable sources with good PTF photometry, 1,822 are sufficiently bright (above the main sequence line) to be plausible members of the North America Nebula complex. Of those, we exclude the 68 sources in the region with r−i > 2 and i < 10 + 2(r−i), including 24 of the 33 sources where we could rule out an infrared excess, as probable giants. This leaves 1,754 variables with photometry consistent with North America Nebula membership.