Characterizing the Stellar Population of NGC 1980

The Orion A molecular cloud contains the nearest regions of massive star formation. Its most active component is the Orion Nebula Cluster (ONC), which contains a few thousands of young stellar objects (YSOs) ranging in mass from the hydrogen burning limit to massive O stars. Extending to the south of the ONC is the L1641 cloud, which contains a somewhat more distributed population of YSOs. NGC 1980 is a cluster found between these two stellar populations. Loosely associated with a massive O9 binary star, ι Ori, this cluster could be an important jigsaw puzzle piece in understanding the star-formation history of the region.

NGC 1980 is thought to be an older cluster than the population of stars that belong to the ONC (<1–3, Myr Da Rio et al. 2010) and L1641 (∼3 Myr, Hsu et al. 2012). Alves & Bouy (2012) have estimated its age to be 4–5 Myr from the lifetime of ι Ori, and later revised the estimate to 5–10 Myr based on the optical colors of the stars in the vicinity of the region (Bouy et al. 2014, hereafter BA14). Pillitteri et al. (2013) performed an X-ray survey of Orion A; they found that toward NGC 1980 there exists an excess of low-extinction sources with ${A}_{V}\sim 0.1$ compared to L1641, and that it also contained one of the largest fractions of YSOs that have already lost their dusty disks, with an age estimate of $\geqslant 5\,\mathrm{Myr}$. More recently, however, Fang et al. (2017) suggested that the age of NGC 1980 is closer in agreement to that of the ONC, with the median age of 1.8 Myr.

Alves & Bouy (2012) and BA14 have questioned the degree to which NGC 1980 is related to the ONC, claiming it to be a foreground population of stars (by as much as 10–40 pc) located at ∼380 pc. Pillitteri et al. (2013) have also argued in support of it, demonstrating that the X-ray luminosities are 0.3 ± 0.1 dex higher in the northern end of L1641 (onto which NGC 1980 is superimposed) compared to the southern end, suggesting that NGC 1980 is 1.4 times closer than the main cloud. However, recently Kounkel et al. (2017) have found a distance toward the ONC of 388 ± 5 pc using stellar parallaxes of non-thermally emitting YSOs; this is a revision to the previously commonly accepted distance of 414 ± 7 pc (Menten et al. 2007). In addition, Kounkel et al. (2017) found the distance toward the southern end of L1641 of 428 ± 10 pc; the difference in X-ray luminosities is most likely caused solely due to the spatial orientation of the cloud. Additionally, radial velocity monitoring of the ONC by Kounkel et al. (2016) and Da Rio et al. (2016) showed that the radial velocities of the stars toward NGC 1980 are indistinguishable from those of the molecular cloud; in fact, this agreement is better than toward any other region in the ONC. A possible argument in favor of NGC 1980 belonging to a foreground population may persist due to the lack of any reflection nebulosity toward ι Ori. However, the degree to which the the ι Ori is related to the cluster is unclear, particularly since this was involved in a dynamical ejection event (de Zeeuw et al. 2001; Gualandris et al. 2004). With these updated distances and radial velocity measurements, the nature of NGC 1980 as a foreground cluster falls into considerable doubt.

NGC 1980 has been left out in many surveys of star formation in Orion that focused on the properties of individual stars. It is located southward of the Trapezium cluster, the heart of the star formation in the ONC, and it is outside of the field of view of the studies by Hillenbrand (1997) and Da Rio et al. (2010). On the other hand, it falls northward of the region covered by the surveys of the L1641 filament (Fang et al. 2009; Hsu et al. 2012, 2013). Similarly to the work by Fang et al. (2017), in this paper, we seek to bridge the gap between these works. We present an analysis of the low-resolution optical spectra of stars identified by BA14 toward this cluster. We asses the youth and determine the spectral types of these stars, as well as measure the equivalent width of the Hα line (Section 2). On the basis of these measurements, we classify these sources as Classical T Tauri Stars (CTTS) and Weak-lined T Tauri Stars (WTTS) based on the definitions from White & Basri (2003). Through analyzing the ratio of CTTS to WTTS, we compare the age of NGC 1980 to that of the ONC and L1641 to confirm its older age (Section 3.1). Finally, we estimate the extinction to these stars (Section 3.2).

We conducted observations using the IMACS spectrograph on the Magellan Baade telescope (Bigelow & Dressler 2003). Observations were done using the $f/2$ camera with the 300 line grism at a blaze angle of 175 and slit width of 0.6". In this configuration, the typical resolution is 4 Å, and the spectral coverage is approximately 4000–9000 Å. However, this coverage may be truncated for the stars close to the edge of the field. Additionally, there is a chip gap ∼100 Å in width that tends to fall in a random place along the spectrum depending on the position of the star within the field of view.

We observed four fields toward NGC 1980 using the multi-slit mode on 2016 January 7. We targeted 280 sources identified by BA14 with $r\lt 21$ mag, of which 212 sources had probabilities of membership (P) greater than 50%. The exposure time was 5 × 10 minutes per field for the first three fields, and the last field had the exposure time of 8 × 10 minutes. The weather conditions deteriorated rapidly over the course of the night, therefore, while the first field had the full sensitivity, the last field, even with the increased exposure time, yielded little in terms of the number of sources detected (Figure 1).

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Partially because of this, we have detected only 148 sources (Figure 2). However, the catalog of BA14 contains many spurious sources. Out of 165 sources that were targeted in the first two fields (of which 134 had $P\gt 50$), only 103 (97 with $P\gt 50$) have been detected. Similar trends persist in the other two fields within the sample limited to the detectable magnitudes. More than 20% of sources with $P\gt 50$, and the vast majority of sources with $P\lt 50$ do not appear to be associated with physical objects, despite falling into the typically detectable magnitude range. Because the catalog of BA14 was compiled from individual frames as opposed to the stacked images, many detections of cross-talks, saturation trails, as well as ghosts and cosmic-rays may be included in the final catalog (Bouy, private communication).

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Several sources also appear to be duplicated in the catalog (some are listed as separate sources up to 3 times with the positional offset of up to 4'', Figure 3). To correct the astrometry to remove this offset, we cross-matched the detected sources with the Gaia DR1 source list (Gaia Collaboration et al. 2016). All but three sources were matched with the DR1 list; for those that were not, we used the astrometry from the initial Gaia source List (Smart & Nicastro 2014). We also re-cross-matched the sources with 2MASS photometry (Cutri et al. 2003) because the positional offset has put many of the sources in BA14 outside of the cross-match radius; therefore, not all sources have their counterpart identified in that catalog. In addition, sources in BA14 that do have JHK photometry listed appear to have a systematic color offset from the 2MASS photometry.

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The IMACS spectra were reduced using the COSMOS pipeline³ following the standard prescription. The final data product does have some issues with the poor subtraction of the nebular emission lines, particularly Hα (6563 Å). In general, this emission originates from across the entire slit, and it is modeled and subtracted in a similar manner to the sky lines for the vast majority of sources. However, in some cases, the nebular emission varies strongly across the slit, making subtraction difficult. This issue has a stronger effect on very low signal-to-noise ratio (S/N) sources. We measured the equivalent width of Hα (${W}_{{{\rm{H}}}_{\alpha }}$) in all spectra with an overall S/N > 5. We visually examined the sky subtracted spectra and flagged the sources with poor subtraction, such as in cases where the flux of Hα does not appear centered and/or confined to the star; in these cases, we do not quote ${W}_{{{\rm{H}}}_{\alpha }}$. To test nebular subtraction and to provide an estimate of the uncertainties of ${W}_{{{\rm{H}}}_{\alpha }}$, we add up all flux along the slit outside of the source (normalizing it by the pixel count) to the flux of the source itself and measure ${W}_{{{\rm{H}}}_{\alpha }}$ again. We do it in two different ways, by measuring a simple total of the flux as is, and by measuring a total of the absolute value of the flux, because, due to variable nebular emission, some of it may be oversubtracted. The uncertainties we report are the average difference between ${W}_{{{\rm{H}}}_{\alpha }}$ we measure and ${W}_{{{\rm{H}}}_{\alpha }}$ with the residuals added. Examples of the Hα line profiles for low, medium, and high S/N data is shown in Figure 4.

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The spectra were analyzed using SPTCLASS⁴ (Hernández et al. 2004) in order to determine the spectral types of the detected stars. This program utilizes three different schemas optimized for three different ranges of stellar masses (K5 and later, late F to early K, and F5 and earlier) based on examining a number of spectral features. For late-type stars (all the sources in this paper were identified as such) the examined features are TiO (${\lambda }_{\mathrm{cen}}=4775$, 4975, 5225, 5475, 5600, 5950, 6255, 6800, 7100, 7150 Å), and VO (${\lambda }_{\mathrm{cen}}=7460$, 7940, 7840, 8500, 8675, 8880 Å) lines. We report only on those spectral types that were obtained using at least three features.

Spectra for some stars have been analyzed in previous works. Spectral types for 25 stars that are in common with Hsu et al. (2012) and 76 stars in common with Fang et al. (2017) show a good agreement within the uncertainties of each survey (usually within one subclass). Same can be said for 13 stars reported on by Da Rio et al. (2010), though due to their use of measurements compiled from various publications utilizing different methodologies, some additional scatter persists. Spectral types for 13 sources that are in common with Rebull (2001) show the largest scatter in the agreement; however, they are typically consistent with each other within the uncertainties (Figure 5). Of the stars presented in this work, 50 do not have pre-existing spectral classifications. We also compare the spectral types we measure to the effective temperature from Da Rio et al. (2016). Assuming typical spectral type to temperature conversions (e.g Pecaut & Mamajek 2013), they also show relatively good agreement with each other.

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We also measured the equivalent widths of the Li i line (6707 Å, W_Li), as it can be used as an indicator of youth and membership of star-forming regions (Briceno et al. 1997). However, given the fact that it is a weak absorption line, due to the poor observing conditions in many sources the S/N was too small for a confident detection. In these cases, the equivalent widths that can be measured from the random noise can be equivalent or greater than W_Li, which is typically expected to be ∼0.5 Å in the young stars. Therefore, we include the upper limits for sources where the Li i line was not detected. We define the primary sample of the members consisting of sources with a confident Li i detection with ${W}_{{Li}}\gt 0.1$ (62 stars), as well as the sources that have been previously identified as YSOs in other studies, have S/N > 5, but it is not sufficiently high enough for a confident Li i detection (43 stars). The remaining 10 sources with ${\rm{S}}/{\rm{N}}\gt 5$ are not included in the list of members; however, they cannot be conclusively rejected as such. Examples of Li i line profiles for low, medium, and high S/N data are shown in Figure 4.

3.1. Accretion

We apply the criteria developed by White & Basri (2003) to separate strongly accreting CTTSs from the WTTSs through analyzing ${W}_{{{\rm{H}}}_{\alpha }}$ depending on the spectral type (Figure 6). Out of 85 stars in our total sample with measured ${W}_{{{\rm{H}}}_{\alpha }}$, 15 are CTTS, for a CTTS fraction (defined as ${N}_{\mathrm{CTTS}}/{N}_{\mathrm{CTTS}+\mathrm{WTTS}}$ of 0.18).

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Pillitteri et al. (2013) have argued that NGC 1980 extends by $\gt 1^\circ$ on the sky, but BA14 confine it to a much more limited area, possibly due to the difference in the methods, since BA14 rely on the background cloud to identify the sources. Nonetheless, we use their density map and define the sources within 015 from the center of the cluster ($\alpha =83\buildrel{\circ}\over{.} 8,\delta =-5\buildrel{\circ}\over{.} 95$ defined by BA14) as members, and the sources outside of that as those that are found on the periphery. There may be bona fide members of NGC 1980 outside of this boundary as well. Nonetheless, this boundary can be used to compare the population that exists at the center of the cluster to its surroundings. At the distance of 388 pc (Kounkel et al. 2017) the cluster diameter of 03 to a physical size of 2 pc. We find that within the cluster the CTTS fraction drops to only 0.09, where only three stars out of 33 show strong accretion signatures. On the other hand, outside of the cluster, the CTTS fraction becomes 0.23 (12 out of 52 stars are accreting), which is dominated by the sources north of the cluster (0.27, 9 out of 33).

Fang et al. (2017) argue that the median age of NGC 1980 is 1.8 Myr. Therefore, the question arises, how significant is the difference that we observe in the CTTS fraction between the two populations. The sample in Fang et al. (2017) is considerably larger, and it covers a bigger area of the ONC to the north, and L1641 to the south, in the $-7^\circ \lt \delta \lt -4^\circ$ range, excluding the Trapezium cluster. The observed targets were also selected from the BA14 catalog; however, there is not a direct 1–1 overlap between the surveys. In their sample, there does appear to be little variation in the CTTS fraction of ∼0.15 throughout the observed regions, whether it is inside NGC 1980 or outside of it. A possible exception to this is the NGC 1981 region, where the fraction does drop to ∼0.09. Fang et al. (2017) argue that the BA14 survey is biased against the sources with the hot inner disks; therefore, the true CTTS fraction should be much higher than what can be observed.

To minimize this bias, we search for sources with Hα observations in other surveys that fall into the NGC 1980 region. We include sources classified by Hsu et al. (2012) and Fűrész et al. (2008), as well as analyze the Hα measurements by Kounkel et al. (2016). In all of the studies, we could identify 107 YSOs in the NGC 1980 region with the Hα line observed, out of which 31 are CTTS, for the total CTTS fraction of 0.29 ± 0.05, assuming Gaussian statistics. Of these sources, 104 can be classified on the basis of the presence of a disk as indicated by infrared excesses detected with the IRAC camera on board the Spitzer Space Telescope (Megeath et al. 2012). Out of 73 WTTSs, only 7 still have a disk, and out of 31 CTTSs, 7 sources do not have infrared excess, which would be indicative of a full disk. The disk fraction for these sources is 0.30 ± 0.05, very similar to what is found by Fang et al. (2017) and Pillitteri et al. (2013).

3.2. Extinction

We estimated the extinction toward the sources in the survey via their 2MASS photometry from comparing the J–H and H–K colors to the intrinsic colors of young (5–30 Myr) stars from Pecaut & Mamajek (2013). We adopt the relations of A_J:A_K = 2.65, A_J:A_K = 1.55 (Megeath et al. 2012), and A_K:A_V = 0.112 (Cardelli et al. 1989). Stars with very low extinctions may result in negative A_V measurements, in those cases A_V is set to 0. We report on the average A_V measured from both J–H and H–K colors for each star in Table 1 (Figure 7). The typical error due to an uncertainty in spectral type is ${A}_{V}\sim 0.1$. We have also attempted to measure A_V through artificially de-redenning the spectra (assuming the standard extinction law R_V = 3.1, Cardelli et al. 1989) until the slope matches that of the template spectra of the corresponding spectral type from the X-Shooter Spectral Library (Chen et al. 2014). While this method does produce mostly comparable A_V measurements, they are also significantly more uncertain.

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Table 1.  Sources Observed toward NGC 1980 with IMACS

BA14	R.A.	Decl.	SPType	${W}_{{{\rm{H}}}_{\alpha }}$ a	WLia	S/N	Accr.	Referenceb	AV	Field
$\#$	J2000.0	J2000.0		Å	Å				mag.
329411	05:34:02.85	−06:05:50.7	M4.8 ± 0.9	−8.9 ± 0.2	0.5	59.4	WTTS	1, 3, 7	0.2	1
331374	05:34:05.99	−06:06:11.7	M4.6 ± 0.6	−53.4 ± 2.7	0.5	66.2	CTTS	1, 3, 5, 7	0.0	1
332392	05:34:07.52	−06:04:46.4	M2.5 ± 0.6	−4.3 ± 0.2	0.4	170.5	WTTS	1, 3, 7	0.0	1
340962	05:34:19.95	−06:08:54.6	M3.3 ± 0.5	−3.5 ± 0.2	0.5	116.9	WTTS	1, 3	0.0	1
341157	05:34:20.26	−05:58:15.6	M4.7 ± 1.4	−5.0 ± 1.2	<0.2	29.0	WTTS	4, 7	0.0	1
344878	05:34:25.89	−06:03:51.9	M5.0 ± 0.5	−14.2 ± 0.6	c	75.0	WTTS	3, 7	0.1	1
346977	05:34:28.84	−06:09:50.4	M5.0 ± 0.7	−7.3 ± 0.4	0.5	35.3	WTTS	1	0.8	1
348520	05:34:30.97	−05:58:03.6	M6.5 ± 1.3	−93.1 ± 33.2	<1.3	5.6	CTTS	5, 6	0.0	1
348805	05:34:31.41	−06:01:54.7	M7.5 ± 5.1	⋯	⋯	4.0	⋯	⋯	⋯	1

Notes.

^aReported only for sources with S/N > 5. ^b(1) This work, (2) Pillitteri et al. (2013), (3) Hsu et al. (2012), (4) Fűrész et al. (2008), (5) Megeath et al. (2012), (6) Morales-Calderón et al. (2012), (7) Fang et al. (2017). ^cLine has either fallen onto the chip gap or off the edge of the field. ^dPoor nebular line subtraction.

Only a portion of this table is shown here to demonstrate its form and content. A machine-readable version of the full table is available.

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In general, approximately half of all sources do have very low extinction with ${A}_{V}\sim 0$. This is true of almost all sources identified as members of NGC 1980, with only six sources (out of 39) with ${A}_{V}\gt 0.1$, and all with ${A}_{V}\lt 1.0$. The degree of extinction toward the cluster comparable to what has been found by Pillitteri et al. (2013) through X-ray observations of the region. On the periphery of the cluster, 32 out of 58 sources have ${A}_{V}\gt 0.1$, 11 of which have ${A}_{V}\gt 1.0$. The Kolmogorov–Smirnov test shows that the two populations are different by 3.5σ.

Most of the sources with very high extinction in our survey originate north of the NGC 1980 cluster, toward the ONC. The A_V distribution of these northernmost sources does not mirror that measured toward the ONC by Da Rio et al. (2010), however, the sample selection of the two surveys is very different and our optical study is biased against highly extincted sources. However, in general our results are consistent with what has been observed in the other works. The extinction in the Orion A molecular cloud is low in the vicinity of the NGC 1980, it rises sharply toward the ONC, and gradually increases southward along the L1641 filament (Da Rio et al. 2010; Hsu et al. 2012; Pillitteri et al. 2013; Fang et al. 2017).

3.3. Li i distribution

As young stars evolve, eventually they will begin depleting Li, and more massive stars will deplete it faster. Therefore, in some cases, the distribution of W_Li as a function of temperature can be used as additional information from which age can be inferred (e.g., Jeffries et al. 2017). We compare the measured W_Li for the sources with firm Li i detections to the expected Li i distribution from the evolutionary tracks from Baraffe et al. (2015). Nearly all of our sample consists of M-type stars; these sources do not begin processing Li i until 5–10 Myr. From this comparison, we can rule out ages older than about 6 Myr for the sources found toward NGC 1980; however, this comparison cannot help to distinguish ages younger than that (Figure 8). A similar distribution of W_Li is found in the surveys by Hsu et al. (2012) and Fang et al. (2017) throughout the ONC and L1641.

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We obtained low-resolution spectra from IMACS toward 148 sources toward NGC 1980, we determined spectral types of these sources, and determined whether or not they can be considered as YSOs on the basis of Li i absorption. Most sources have ${A}_{V}\sim 0$, in agreement with what have been found by Pillitteri et al. (2013) and BA14. We determined whether or not these sources have Hα excess due to the ongoing accretion and searched for sources that fall into the region identified as NGC 1980 with Hα measurements in the previous works. We found that 31 out of 107 stars in the cluster can be identified as accretors, for a CTTS fraction of 0.29 ± 0.05.

As the population ages, the CTTS fraction decreases (e.g., Mamajek 2009, C. Briceño et al. 2017, in preparation). Within the ONC, containing a large population that spans in age between $\lt 1\mbox{--}3\,\mathrm{Myr}$, the CTTS fraction is ∼0.5 (Fűrész et al. 2008; Da Rio et al. 2009). Within the outlying L1641 cloud it decreases to is 0.35 (∼3 Myr; Hsu et al. 2012). Within the Ori OB1b sub-association (4 Myr), CTTS fraction is 0.13, and only 0.06% toward the 25 Ori (8 Myr) comoving group. (Briceño et al. 2007).

From this, it does appear that the population of NGC 1980 is indistinguishable from the population of L1641. This is similar to what has been observed by Fang et al. (2017). However, the CTTS fraction is considerably lower in this cluster than in the ONC itself. It is possible that the sample is highly incomplete. Given that all classifications were conducted from optical surveys, there exists an inherent bias against deeply embedded and accreting sources. On the other hand, WTTSs typically have a much smaller IR excess, making them harder to identify as targets when planning a survey. Since the extinction in the vicinity of NGC 1980 is much lower than in the ONC, it would be unlikely for there to be a larger population of highly reddened undetected sources to be present in NGC 1980 over the ONC. Unless the CTTS fraction is significantly lower in the ONC due to a presence of previously unanalyzed WTTSs, NGC 1980 is probably somewhat older. It is unlikely to be as old as 5 Myr (the age that was previously estimated by BA14). We estimate its age to be ∼3 Myr, similar to L1641. A possible reason why Fang et al. (2017) do not see the difference between the ONC and NGC 1980 is because the sample for which they determined the ages is highly incomplete near the ONC (Trapezium and OMC 2/3) region, the youngest and most deeply embedded population. Furthermore, with the updated distances from Kounkel et al. (2017), the ages measured by Fang et al. (2017) would be ∼20% older, which would also help reconcile some of the differences.

Because this work does not explicitly measure distances to the stellar populations, we cannot conclusively confirm or disprove whether NGC 1980 is in the foreground of the ONC or not. The reason for low extinction near the region could be that the cluster is located on the outer layer of the molecular cloud, in which case it would technically not be incorrect to refer to the cluster as a foreground population by the slimmest of margins. However, a more likely explanation would be that the stellar winds have cleared out the molecular gas, there could have been less gas near the cluster than there was near the ONC from the very beginning. Kounkel et al. (2017) have shown that the ONC is found at the same distance that was suggested by BA14 for NGC 1980. Kounkel et al. (2017) did not detect any of the stars toward NGC 1980 in their sample; therefore, they were not able to estimate stellar parallax toward the region. The distance found by BA14 was found photometrically; therefore, it has significantly larger uncertainty in its value.

Until the uncertainties in the distance are smaller than any distance offsets, it is not entirely meaningful to discuss substructure, as well as what is and what is not in the foreground. The future data releases from the Gaia space telescope should be instrumental in measuring the three-dimensional relations and intercluster kinematics in the region, and it should resolve this matter once and for all. Its observations should shed light on the initial conditions that have led to the current configurations of these star-forming regions.

The authors would like to thank the LCO operators and staff for their help on the observing run. M.K. acknowledges the insightful conversation with Min Fang. We also acknowledge a prompt and helpful report from an anonymous referee. This research made use of the cross-match service provided by CDS, Strasbourg. This work was supported in part by the University of Michigan.

Facility: Magellan:Baade(IMACS).

Software: SPTCLASS (Hernández et al. 2004), TOPCAT (Taylor 2005), COSMOS (http://code.obs.carnegiescience.edu/cosmos).