Open Cluster IC 1369 and Its Vicinity: Multicolor Photometry and Gaia DR2 Astrometry

IC 1369 is an open cluster located at R.A. = 21:12:10, decl. = +47:46 in the direction of the association Cygnus OB7. The cluster drew our attention after the burst of the Nova Cygni (V1500 Cyg) in 1975 located 24' north of it. In the 2° × 2° area around the Nova, 52 stars mostly of early spectral classes were measured photoelectrically in the seven-color Vilnius photometric system, and the run of the interstellar extinction with distance was estimated up to 7 kpc (Straižys et al. 1979). It was shown that the interstellar extinction versus distance plot in the direction of Nova and the cluster IC 1369 is flat between 0.5 and 2 kpc, and this result was unexpected for the area lying close to the Galactic plane in Cygnus. Until the Gaia era, the cluster itself has not been investigated photometrically applying high-accuracy photoelectric or CCD techniques. Its distance was estimated only in a few photographic studies published by Trumpler (1930), Dibai (1958), and Hassan (1973)—the results are strongly different, and they cover the range from 1500 to 7700 pc. The color excess and age of the cluster were also estimated very approximately. Kharchenko et al. (2013) estimated the cluster parameters using infrared JHK_s photometry from the Two Micron All Sky Survey (2MASS) catalog. The values d = 2530 pc, E_B−V = 0.57, and the age 350 Myr were obtained. Ahumada & Lapasset (1995, 2007), using the Hassan (1973) photographic UBV photometry, identified six blue stragglers in the cluster.

Recently, the distance to the cluster, 3436 pc (DM = 12.68 mag), was determined by Cantat-Gaudin et al. (2018) using trigonometric parallaxes taken from the Gaia Data Release 2 (hereafter DR2). To calculate the membership probabilities, the Gaia proper motions and parallaxes have been used. Bossini et al. (2019) used an automatic determination of cluster parameters (age, distance, and extinction) through a Bayesian isochrone fitting of the observed Gaia magnitudes G, G_BP, and G_RP for the high-probability cluster member stars. They obtained that the cluster is located at a distance of 3221 pc (DM = 12.54 mag), its age is 288 Myr, and the extinction A_V = 2.05 mag.

Consequently, IC 1369 is an object of the Perseus arm, one of the most distant clusters located in the rather transparent dust window close to the Galactic plane. Its extinction is the result of dust clouds located in the whole crossing length of the Local spiral arm and the space between the Local and Perseus arms. For investigation of the extinction in this direction we use deep CCD photometry of stars in the Vilnius medium-band photometric system with the mean wavelengths 345 (U), 374 (P), 405 (X), 466 (Y), 516 (Z), 544 (V), and 656 (S) down to V = 20 mag, which allows for the photometric two-dimensional classification of stars and extinctions down to V = 19 mag. Thus, we can get interstellar reddenings and extinctions for the cluster members individually, and this fact is important in plotting a more realistic HR diagram of the cluster. For the identification of cluster members we use the coordinates, parallaxes, and proper motions of stars from the Gaia DR2 and the Bovy et al. (2009) method. One more exceptional property of IC 1369 is the presence in it of six suspected blue stragglers (Ahumada & Lapasset 1995, 2007), and this fact needs to be verified. At last, we also tried to verify the presence of a transparent window in the direction of IC 1369 behind the northern extension of the Great Cygnus Rift (Straižys et al. 1979). All this makes the cluster IC 1369 and its vicinity worth of a special investigation with modern photometric and astrometric methods.

The investigated 13' × 13' area (Figure 1) is centered on the cluster IC 1369 and it covers the whole cluster whose size is about 7' × 9', see Section 5. CCD exposures of different durations with the filters of the Vilnius system were obtained on 2014 November 19–23 with the 1.8 m VATT telescope on Mt. Graham, Arizona using the STA0500A CCD camera and a 4 k × 4 k chip. The exposure lengths were from 10 to 1200 s in the visible passbands and from 50 to 1500 s in the ultraviolet passbands. Processing of CCD frames was done with the Image Reduction and Analysis Facility (IRAF) program package in the point-spread function (PSF) mode.

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The investigated area had no standard stars of the Vilnius system, so the transformation equations of V magnitudes and six color indices from the instrumental to the standard system were obtained using tie-in observations of the IC 1369 field to the cluster NGC 6997 in the North America Nebula investigated in the same photometric system by Zdanavičius & Straižys (1990) and Laugalys et al. (2006). The choice of this cluster is based on its proximity to IC 1369: both clusters are separated only by 4°. Since the response curves of the instrumental and standard systems are very close, the coefficients at the color terms of transformation equations are close to 1.0.

The catalog of stars in the IC 1369 area in the standard Vilnius system (Table 1) contains 2694 stars down to V ≈ 20 mag with the following results: star number; equatorial coordinates J2000; magnitude V; six color indices, U–V, P–V, X–V, Y–V, Z–V, and V–S; their uncertainties, e_V, e_(U−V), e_(P−V), e_(X−V), e_(Y−V), e_(Z−V), and e_(V−S); and photometric spectral types in the Morgan-Keenan (MK) system. The uncertainties take into account the measurement errors and the errors of transformation to the standard system, which are of the order of ±0.02 mag. At V = 17 mag the uncertainties for color indices X–V, Y–V, Z–V, and V–S are mostly lower than 0.03 mag, and for the ultraviolet indices U–V and P–V they are mostly lower than 0.04 mag. For the stars at V = 19 mag the uncertainties of X–V, Y–V, Z–V, and V–S are lower than 0.05–0.06 mag, and for the indices U–V and P–V they are lower than 0.08–0.09 mag. A few percent of stars exhibit even larger uncertainties due to optical duplicity, variability, or insufficient signal-to-noise ratio, and for them the ultraviolet color indices are not given.

For the two-dimensional classification of stars the updated QCOMPAR code, described in our previous publications (e.g., Straižys et al. 2013), was applied. It makes a matching of 14 different interstellar reddening-free Q-parameters of a program star to those of about 10,000 stars from the Vilnius photometric catalog of photoelectric observations (Straižys & Kazlauskas 1993), hereafter—the standard catalog, supplemented by the Gaia DR2 (Gaia Collaboration et al. 2018) and Hipparcos (Perryman et al. 1997) parallaxes (for the brightest stars) and the distances from Bailer-Jones et al. (2018). The parallaxes were used only in the cases when the distances in Bailer-Jones et al. (2018) were absent. We use the following Q-parameters: Q_UPY, Q_UPYV, Q_UXY, Q_UYV, Q_PXYV, Q_PYV, Q_PYZ, Q_XYZ, Q_XYV, Q_XZV, Q_XZS, and Q_YZV, which are sensitive to spectral classes, luminosities, and peculiarities in different temperature ranges, see Straižys (1992). The match quality is characterized by

$$\begin{eqnarray}&&{\sigma }_{Q}=\sqrt{\tfrac{{\sum }_{n}^{}{\rm{\Delta }}{Q}_{i}^{2}}{n}},\end{eqnarray} \tag{ 1 }$$

where ΔQ are differences of corresponding Q-parameters of the program star and the standard star and n is a number of the compared Q-parameters. The standard catalog contains stars of various spectral and luminosity classes, metallicities, and peculiarities. The Gaia data allow the refinement of the absolute magnitudes and luminosity classes of MK types for all stars of the Vilnius standard catalog and the catalog of the program stars, taking into account the absolute magnitudes derived from the observed V magnitudes, distances, and calculated values of interstellar extinction. For every program star the code selects up to 10 stars from the standard catalog with the least σ_Q. The digitized spectral types of the selected stars are averaged with weights that take into account the σ_Q values. All stars in the investigated area have the Bailer-Jones et al. (2018) distances, so we have used them instead of the Gaia parallaxes.

For the classification of stars selected to be the cluster members (see below), two more classification codes were used: (1) the same QCOMPAR code based on the intrinsic color indices of stars of various spectral and luminosity classes, instead of the list of standard stars; and (2) the NORMA code based on the intrinsic color indices and interstellar extinctions described in Straižys et al. (2018).

The classification accuracy was recently described by Straižys et al. (2018): the accuracy of spectral class is of the order of 1–2 decimal subclasses and the accuracy of the luminosities for B8-A-F-G5 stars of luminosity classes V–IV–III is about one luminosity class. For K-type stars, the accuracy is about 0.5 of spectral subclass and 0.5 of luminosity class. For the classification of K- and M-type stars the ultraviolet U–V and P–V color indices are not essential since the photometric temperature and luminosity criteria are sufficient in the passbands from X to S.

Color excesses of stars are calculated by the equation

$$\begin{eqnarray}&&{E}_{Y-V}={\left(Y-V\right)}_{\mathrm{obs}}-{\left(Y-V\right)}_{0},\end{eqnarray} \tag{ 2 }$$

where (Y − V)₀ are the intrinsic color indices taken from Straižys (1992). The E_Y−V values are transformed to the extinctions A_V by the equation

$$\begin{eqnarray}&&{A}_{V}=4.16\,{E}_{Y-V},\end{eqnarray} \tag{ 3 }$$

where the coefficient 4.16 corresponds to the normal extinction law justified by the ratio E_J−H/E_H−K, which is found in the investigated area to be about 2.0, which is close to the value for the normal extinction law found in the publications by Straižys & Laugalys (2008) and Straižys et al. (2014, 2015, 2016). Typical uncertainties of A_V due to the observational errors of Y–V and the errors of the intrinsic (Y − V)₀ is ∼0.10 mag for the stars with V < 17 mag and ∼0.20 mag for V between 17 and 19 mag. The extinction error mostly depends on the errors of spectral classes, while the error of the luminosity class is much less important. Among the 143 cluster members with the membership probability ≥0.75, we identified 17 metallic-line (Am) stars and one peculiar (Ap) star. These types of stars have been indicated either by the QCOMPAR code or identified by the reddening-free photometric parameter Q_XYV (see Straižys 1992, Figure 105). Thus, for the investigation of the extinction we used 544 field stars of spectral classes O–G with the classification errors ≤0.035 mag, and 98 stars of spectral classes K–M with the classification errors ≤0.050 mag, as well as 123 cluster members (without Am, Ap stars, and two stars with lower classification accuracy).

Distances to stars were taken from the catalog of Bailer-Jones et al. (2018) where they have been calculated from the Gaia DR2 parallaxes inverted to distances, taking into account nonlinearity of the transformation and asymmetry of the resulting probability distribution.

Members of the cluster were selected using the extreme deconvolution program from Bovy et al. (2009, 2011).⁵ Clustering was performed using the five parameters from Gaia DR2: equatorial coordinates, parallaxes, and proper motions in R.A. and decl. in the 18' × 18' region with the center given by WEBDA. The cluster completely fits in our 13' × 13' field, as it is found lower in this section. As the cluster membership probabilities of stars, p, we used the hyperparameter q_ij introduced by Bovy et al. (2009; Equation (16)). The cluster strongly peaks against the field stars in the proper motion diagram (see Figure 2). We have found 168 possible cluster members with probabilities p ≥ 0.75. This cutoff corresponds to a sharp rise of probability of all possible cluster members (see Figure 3). After the rejection of the stars fainter than V = 19 mag and the stars with uncertain classification, this number has decreased to 143. The rectangle in the left lower corner of Figure 2 is enlarged in Figure 4 showing the associated proper motion uncertainties. The vector point diagram μ_α versus μ_δ for these stars is plotted on the contour map. The cluster center is denoted by an open circle. Its coordinates are μ_α = −4.6431 ± 0.0059 mas and μ_δ = −5.6219 ± 0.0056 mas.

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Figure 5 shows the histogram for distances from Bailer-Jones et al. (2018) for the 143 cluster members with the membership probability ≥0.75. The simple mean of the distances of the 143 cluster members is 3280 pc and the standard deviation is σ = 500 pc. The corresponding distance modulus DM = V–M_V of the cluster is 12.58 ± 0.32 mag.

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The cluster distance was also estimated by applying the maximum-likelihood method for Gaia DR2 parallaxes (Equation (1) of Cantat-Gaudin et al. 2018). After the addition of the systematic parallax error of +0.029 (Lindegren et al. 2018), the mean parallax of the cluster is found to be 0.294 ± 0.044 mas. The corresponding distance to the cluster is ${3370}_{-410}^{+540}$ pc and DM = 12.64 ± 0.30 mag.

The center of IC 1369 for the stars with the membership probability ≥0.75 is at R.A. (2000) = 21:12:07.63, decl.(2000) = +47:46:10.5. The cluster size is 72 in R.A. and 89 in decl. At a distance of 3280 pc, the linear size is 6.9 × 8.5 pc.

Figure 6 shows the physical diagram log L/L_⊙ versus log ${T}_{\mathrm{eff}}$ for the 123 cluster members of spectral classes B5-A-F-G from Table 2. These stars (except for red giants) have an uncertainty of their classification ≤0.035 mag. All stars fainter than V = 19.0 mag as well as 18 metallic-line and peculiar stars are rejected since their spectral classes and, consequently, intrinsic color indices and extinctions are of insufficient accuracy.

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The isochrones for the 300 and 350 Myr ages are taken from the Padova database of stellar isochrones for the solar metallicity (Bressan et al. 2012).⁶ The solar metallicity is based on the Apache Point Observatory Galactic Evolution Experiment (APOGEE) spectroscopic investigation (Frinchaboy et al. 2013; Ness et al. 2016) of the three red giants (Nos. 819, 1534, and 1680) for which we have estimated large membership probabilities and whose distances fall in the range of 3340–3540 pc. Among the cluster members there are no more stars with APOGEE metallicities. The Zero Age Main Sequence (ZAMS) line was drawn through the unevolved portions of the isochrones corresponding to 1, 50, and 100 Myr. Luminosities of stars in solar units were calculated with the equation

$$\begin{eqnarray}\begin{array}{rcl}\mathrm{log}L/{L}_{\odot } & = & 0.4({M}_{\mathrm{bol},\odot }-{M}_{\mathrm{bol},\star })\\ & = & 0.4(4.72-{V}_{0}+\mathrm{DM}-\mathrm{BC}),\end{array}\end{eqnarray} \tag{ 4 }$$

where V₀ = V − A_V is the intrinsic magnitude of the star, ${M}_{\mathrm{bol},\odot }=4.72$ is the absolute bolometric magnitude of the Sun, M_bol,⋆ = M_V + BC is the absolute bolometric magnitude of the star, BC is its bolometric correction, and DM is the true DM of the cluster. The DM = 12.58 mag is taken. This distance seems a little too large since almost all A7–F8 main-sequence stars lie above the ZAMS line, i.e., they are too bright. We cannot exclude that in reality the cluster distance is somewhat smaller than it is given by the Gaia DR2 parallaxes, even after their correction by +0.029 mas as suggested by Lindegren et al. (2018). The possible presence of regional systematic errors of parallaxes up to 0.1 mas in the Gaia DR2 has been discussed by Lindegren et al. (2018), Arenou et al. (2018), and Cantat-Gaudin et al. (2018). More arguments that the Gaia DR2 distances at d > 2 kpc are too large are given by Zinn et al. (2019), Khan et al. (2019), Leung & Bovy (2019), and Chan & Bovy (2019) applying the asteroseismologic and spectrophotometric distances. To get a better position of the main-sequence stars with respect to ZAMS in Figure 6, we have to increase the Gaia parallax by about 0.08 mas. Then the distance to the cluster becomes ∼2900 pc. The necessity of a revision of the Gaia parallaxes and distances prevents the use of a more sophisticated method for the age estimation than the fit by eye.

Effective temperatures and bolometric corrections for the B-A-F stars of luminosities V–III are taken from Flower (1996) and the G5–G8 giants of luminosities II–III are from the monograph Straižys (1992), Appendix 3 (interpolation between luminosities III and I–II). At $\mathrm{log}{T}_{\mathrm{eff}}$ = 4.0 and 3.8 (spectral types A0 V and F8 V) the error crosses for 1σ are given. They are based on Equation (3), taking the uncertainties 0.02 mag in V, 0.12 mag in A_V, and for BC they correspond to ±1 subclass in spectral type. The uncertainties of T_eff also correspond to the ±1 subclass errors. The vertical error bars in the crosses do not take into account uncertainty of the cluster distance modulus discussed in the previous paragraph. The red broken line between $\mathrm{log}{T}_{\mathrm{eff}}$ = 3.8 and 3.9, located above the ZAMS line by ΔM_V = 0.75 mag or ΔL/L_⊙ = 0.3, designates the upper limit of binary cluster members.

The isochrones for the ages 300 and 350 Myr give the best-fit "by eye" to the evolved cluster members of spectral classes B9–A with the evolutionary deviation and red giants. A more precise estimation of the age is hardly possible owing to the possible systematic distance error and the scatter of stars on the deviated sequence. A few A and F stars (like No. 905) deviate upwards from the ZAMS line and isochrones too much to be explainable by a possible binarity. We suspect that they are foreground stars, although their Gaia distances and proper motions fall into the range of real cluster members.

Eight red giants of spectral classes G5–G8 and photometric luminosity classes between II and III are probable cluster members. However, two of them (No. 2193 and 2214) have a low classification accuracy and in Figure 6 are not plotted. Five giants (Nos. 819, 1097, 1128, 1534, and 1549) lie close to the 300–350 Myr isochrones. One giant (No. 1680) is located about 0.4–0.5 higher than the others—it can be a physical binary consisting of two red giants. Only one possible blue straggler of spectral class B5 (No. 1284) is present. Its V = 13.89 mag, membership probability p = 0.99, and d ≈ 3260 pc. Six stars in the area of IC 1369, included into the catalog of blue stragglers by Ahumada & Lapasset (1995, 2007), are field A-type stars and do not belong to the cluster.

Figure 7 shows the interstellar extinction A_V versus the Gaia DR2 distance from Bailer-Jones et al. (2018) in the investigated area up to 5.5 kpc. The 544 field stars of spectral classes O–G are plotted in green, the 98 field stars of spectral classes K–M in red, and 123 cluster members in blue. For the field stars, one more restriction was added: all the used stars have the accuracy of parallaxes <25%. The Am and Ap stars are not plotted. Panel (b) shows the same stars with the averaged A_V values with a step of 200 pc and their standard deviations, as well as the average extinction of the cluster IC 1369.

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It is evident that the first extinction rise up to 1.5–2.0 mag happens at 300–700 pc. This rise may be related to outskirts of the dark cloud LDN 970 = Barnard 361, located ∼25' to south, and the northern extension of the Great Cygnus Rift. According to the summary paper by Hilton & Lahulla (1995), the cloud LDN 970 is located at 350–400 pc. The northern extension of the Great Cygnus Rift surrounding the North America and Pelican nebulae is located at 520 ± 50 pc (Straižys et al. 2015). Behind the Rift, the rise of the extinction is quite small, and at 2.0 kpc our sight line leaves the Local spiral arm (see Valée 2005). Approaching the Perseus arm between 2.5 and 3.0 kpc, the extinction jumps again up to 2.5–3.5 mag. Since the cluster distance is close to 3.3 kpc, due to parallax errors, the apparent distances of the cluster members (blue dots) cover distances between 2.5 and 4.5 kpc. The extinction of the cluster members covers the range from 2.1 to 3.0 mag, the average value being 2.54 ± 0.21 mag. The average position of the cluster members within the error cross is in agreement with the extinction of the field stars.

The variability of the extinction in the cluster region requires that each star before being plotted in the HR diagram (Figure 6) must be dereddened individually. Collective dereddening gives a larger scatter of the extinction values and can lead to an age determination of lower accuracy.

The run of the extinction with distance (Figure 7) is in a good agreement with the A_V versus d plot published by Straižys et al. (1979) and based on photoelectric photometry of 52 stars in the Vilnius system. The transparent corridor between 1 and 3 kpc in this direction is also well seen in the Gaia–2MASS 3D maps of the galactic dust within 3 kpc from the Sun published recently by Lallement et al. (2019).

The extinction run with distance can be obtained with the Argonaut calculator (Green et al. 2014, 2015, 2018, 2019) based on a statistical method applying stars observed in the Panoramic Survey Telescope and Rapid Response System (Pan-STARRS1), 2MASS, and Gaia surveys and a model of distribution of stars in the Galaxy.⁷ In general, the Argonaut extinction curve versus distance is in agreement with our Figure 7, but the extinction values calculated with the Bayestar19 calculator are about 0.3–0.5 mag lower than our average extinctions. We consider that the main reason of this disagreement is hidden in the star distribution model accepted by the Argonaut method, which is based on old stars where all main-sequence stars are cooler than about F5 (Green et al. 2014). We have compared our photometric spectral types with the intrinsic colors (g − r)₀ = (g − r) − E_gr based on the Pan-STARRS1 photometry (Chambers et al. 2019) and the Argonaut data.⁸ The result is a complete absence in the Argonaut lists of stars with ${(g-r)}_{0}\lt 0.20$, corresponding to B, A, and early-F stars: all of them are shifted into the F5–G domain with (g − r)₀ > 0.2. This is well seen in Figure 8 where the Argonaut absolute magnitudes are plotted against their intrinsic colors (g − r)₀. The lower branch contains main-sequence stars of spectral classes F5 to K7. The mean values of their intrinsic colors in the Pan-STARRS1 system for F5, G0, G5, K0, K5, and K7 dwarfs, shown in Figure 8, were obtained using color indices g − r from Chambers et al. (2019) and MK spectral types from our studies of the areas in the direction of the cluster M67 (Boyle et al. 1998; Laugalys et al. 2004) and the North Ecliptic Pole (Zdanavičius et al. 2012), both at galactic latitudes of 30°–32°. Small corrections for interstellar reddening were taken into account. The upper branch in Figure 8 contains subgiants and giants of spectral classes G and K.

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It is evident that in Figure 8 the cluster main-sequence stars (B8-F8, blue dots) overlap the subgiant and giant sequence, instead of being located at the left extension of the main sequence, up to (g − r)₀ = −0.3. The same situation is also present in the directions of the clusters M29 and IC 4996 in Cygnus, investigated in our previous papers (Straižys et al. 2015, 2019), thus this result is a property of the Argonaut method, which has no possibility to recognize B and A stars in the presence of interstellar reddening without ultraviolet magnitudes, see Straižys et al. (1998). As a result, color excesses (and extinctions) of B–A–F5 stars have been found considerably lower. Another systematic error in color excesses (and intrinsic colors) is seen on the main sequence: instead of being concentrated at the corresponding spectral classes, F5–G5 dwarfs cover wide ranges of (g − r)₀ between 0.22 and 0.50 along the same sequence. One more possible source of the systematic errors in the Argonaut color excesses can be the neglect of the bandwidth effect since Pan-STARRS1, 2MASS, and Gaia systems have very broad passbands with strong dependence of color excesses and extinctions on spectral type (or temperature). Also, in the direction of IC 1369, the Argonaut calculator gives the median metallicity [Fe/H] = −0.3 dex, while the APOGEE survey gives the metallicity close to the solar one. This result follows from a comparison of metallicities of 12 stars common in both catalogs in the 18' × 18' region centered on the cluster. The lower metallicity shifts the Argonaut stars to fainter absolute magnitudes M_r and to lower extinctions. The detected systematic errors require a special investigation covering a larger variety of the galactic coordinates and a new calibration of the intrinsic Pan-STARRS1 colors in temperatures and MK spectral types.

Employing the coordinates, parallaxes, and proper motions from the Gaia Data Release 2, we identified 143 members of the cluster IC 1369 having membership probabilities ≥0.75. The distance to the cluster is found by two methods. The method of fitting a Gaussian curve to the histogram of distances of 143 members from Bailer-Jones et al. (2018) gives 3280 ± 500 pc. The method of average parallaxes (Cantat-Gaudin et al. 2018) for the same stars gives ${3370}_{-410}^{+540}$ pc. Thus we confirm that the cluster is located at the inner edge of the Perseus spiral arm. Our by-eye fit of the main-sequence stars to the ZAMS line in the physical HR diagram requires an increase of the parallax by ∼0.05 mas, in addition to the shift by 0.029 mas found by Lindegren et al. (2018). After the addition of ∼0.08 mas, the cluster distance becomes close to 2900 pc. A possibility of regional systematic errors in the Gaia DR2 parallaxes of similar order have been discussed in several investigations (Arenou et al. 2018; Cantat-Gaudin et al. 2018; Lindegren et al. 2018; Chan & Bovy 2019; Khan et al. 2019; Leung & Bovy 2019; Zinn et al. 2019).

From photometry of the cluster and the field stars down to a magnitude of V ∼ 19 mag in the Vilnius seven-color photometric system, their MK types (spectral and luminosity classes) and interstellar extinctions were obtained. For the plot of the A_V versus distance, 642 field stars and 123 cluster members with the best classification accuracy were used. The extinction shows a steep rise up to 1.5–2.0 mag at 300–700 pc, which probably is related to the nearby dark cloud LDN 970 at 350–400 pc and the northern extension of the Great Cygnus Rift at 520 pc. The second jump of the extinction up to 2.5 mag is seen at the beginning of the Perseus arm at 3.0 kpc. Between 0.8 and 2.5 kpc the average extinction remains at a constant value of 1.7 ± 0.3 mag. This extinction run with distance is in good agreement with Straižys et al. (1979) but our extinctions in the area are ∼0.3–0.5 mag larger than those given by the Argonaut calculator (Green et al. 2018). Probably, the main reason of this difference is the acceptance in the Argonaut method of a star distribution model, which is based on old stars cooler than about F5 and having lower metallicities. The average extinction of the cluster members is 2.54 ± 0.21 mag. If the cluster is located close to 3.3 kpc, its size is 6.9 (R.A.) and 8.5 pc (decl.).

After individual dereddening of the cluster members with the most accurate classification, we calculated their luminosities in solar units and plotted a physical HR diagram with the coordinates $\mathrm{log}L/{L}_{\odot }$ versus $\mathrm{log}{T}_{\mathrm{eff}}$, which results in a by-eye estimate of the cluster's age close to 300–350 Myr. The hottest stars of the evolved part of the main sequence are of spectral classes B9. Among the members six G5–G8 giants of luminosity class II–III are present. One possible blue straggler of spectral class B5 IV is identified. The other six stars included in catalog of blue stragglers by Ahumada & Lapasset (1995, 2007) are not cluster members. The cluster also contains at least 17 Am stars and one Ap star.

A by-product of the investigation is the catalog of 2694 stars down to V ≈ 20 mag with the results of seven-color photometry. For about 1390 stars down to V = 19 mag two-dimensional spectral types in the MK system are obtained, while for the fainter stars one-dimensional spectral classes are given. These results have been obtained for the first time, and they may be useful in the future.

This work has made use of data from the European Space Agency (ESA) mission Gaia (https://www.cosmos.esa.int/gaia), processed by the Gaia Data Processing and Analysis Consortium (DPAC, https://www.cosmos.esa.int/web/gaia/dpac/consortium). Funding for the DPAC has been provided by national institutions, in particular the institutions participating in the Gaia Multilateral Agreement. The use of the Simbad (CDS), WEBDA (Masaryk University), and SkyView (NASA) databases is also acknowledged. We also thank the anonymous referee for valuable comments that helped to improve the manuscript. Preliminary results of this investigation were presented at the AAS Meeting No. 233 (Boyle et al. 2019). The project is partly supported by the Research Council of Lithuania, grant No. S-MIP-17-74.