Candidate planetary nebulae in the IPHAS photometric catalogue
K. Viironen, R. Greimel, R. L. M. Corradi, A. Mampaso, M. Rodriguez, L. Sabin, G. Delgado-Inglada, J. E. Drew, C. Giammanco, E. A. Gonzalez-Solares, M. J. Irwin, B. Miszalski, Q. A. Parker, E. R. Rodriguez-Flores, A. Zijlstra
aa r X i v : . [ a s t r o - ph . GA ] J un Astronomy&Astrophysicsmanuscript no. viironen09b c (cid:13)
ESO 2018October 31, 2018
Candidate planetary nebulae in the IPHAS photometric catalogue
K. Viironen , R. Greimel , R. L. M. Corradi , A. Mampaso , M. Rodr´ıguez , L. Sabin , G. Delgado-Inglada J. E.Drew , C. Giammanco , E. A. Gonz´alez-Solares , M. J. Irwin , B. Miszalski , , Q. A. Parker , E. R.Rodr´ıguez-Flores , , and A. Zijlstra Instituto de Astrof´ısica de Canarias (IAC), C / V´ıa L´actea s / n, 38200 La Laguna, Tenerife, Spaine-mail: [email protected] Institut f¨ur Physik, Karl-Franzens Universit¨at Graz, Universit¨atsplatz 5,8010 Graz, Austria Instituto Nacional de Astrof´ısica, ´Optica y Electr´onica INAOE, Apdo Postal 51 y 216, 72000 Puebla, Pue., Mexico Jodrell Bank Centre for Astrophysics, Alan Turing Building, University of Manchester, Manchester, M13 9PL, UK Centre for Astrophysics Research, University of Hertfordshire, College Lane, Hatfield AL10 9AB Institute of Astronomy, Madingley Road, Cambridge CB3 0HA Department of Physics, Macquarie University, Sydney, Australia Observatoire Astronomique, Universit´e de Strasbourg, 67000, Strasbourg, France Instituto de Geof´ısica y Astronom´ıa, Calle 212, N. 2906, CP 11600, La Habana, Cuba
ABSTRACT
Context.
We have carried out a semi-automated search for planetary nebulae (PNe) in the INT Photometric H-Alpha Survey (IPHAS)catalogue. We present the PN search and the list of selected candidates. We cross correlate the selected candidates with a number ofexisting infrared galactic surveys in order to gain further insight into the nature of the candidates. Spectroscopy of a subset of objectsis used to estimate the number of PNe present in the entire candidate list.
Aims.
The overall aim of the IPHAS PN project is to carry out a deep census of PNe in the northern Galactic plane, an area where PNdetections are clearly lacking.
Methods.
The PN search is carried out on the IPHAS photometric catalogues. The candidate selection is based on the IPHAS and2MASS / UKIDSS colours of the objects and the final candidate selection is made visually.
Results.
From the original list of ∼
600 million IPHAS detections we have selected a total of 1005 objects. Of these, 224 are knownobjects, leaving us with 781 PN candidates. Based on the initial follow-up spectroscopy, we expect the list to include very young andproto-PNe in addition to genuine, normal PNe ( ∼ Key words.
Surveys – ISM: planetary nebulae: general – Stars: binaries: symbiotic
1. Introduction
Planetary nebulae (PNe) are the evolutionary end products ofmost low and intermediate mass stars (approximately 1 to 8solar masses). About 2700 PNe have been detected so far inthe Galaxy, including the objects listed by Acker et al. (1994,1996) and the new PNe found in the Southern Hemisphere bythe AAO-UKST H α Survey (Parker et al. 2005; Miszalski et al.2008). However, the expected Galactic PN population is muchlarger, with estimates varying from 28000 ± ± < . ffi culties in observ-ing nebular objects in areas of high extinction and where confu-sion with other types of nebulae (e.g. H ii regions) is significant. Send o ff print requests to : K. Viironen IPHAS is the northern counterpart of the AAO-UKST H α Survey which mapped the Southern Galactic plane ( | b | ≤ ∼ ff use, extended emission. However, theresolution and photometric quality of IPHAS is better, makingit more suited to search for compact PNe.IPHAS mapped 1800 degrees of the Northern GalacticPlane (a band between b = –5 to + α filter (centralwavelength and width: 6568 Å /
95 Å) and two Sloan filters ( r ′ and i ′ ) were used for matched 120, 30, and 10 s exposures, re-spectively. For point sources the survey covers the magnituderange from r ′ ∼
13 to 22 mag (5 σ detection limit, magnitudesare in the Vega system) with the median magnitude limit being r ′ =
21 mag. Each IPHAS field is observed twice at two closely
K. Viironen et al.: Candidate planetary nebulae in the IPHAS photometric catalogue overlapped pointings. More information about the survey can befound in Drew et al. (2005).To search for new PNe in IPHAS, we use two search meth-ods. A semi-automated method was used based on IPHAS pho-tometry to find mainly small angular diameter PNe (typically ≤ ′′ ), and visual inspection of IPHAS mosaics to find mainlyextended PNe (typically ≥ ′′ ). The two methods overlap in thesize of objects found, leading to a complete search for PNe withdiameters from subarcseconds to many arcminutes. Here wepresent the search method and the results of the semi-automatedPN search. The results of the visual PN search will be publishedin the near future (Corradi et al. 2009; Sabin et al. 2009, S09from now on).A search for barely resolved or point-like PNe based on acatalogue of 4853 H α emitting stars (Witham et al. 2008) waspublished in Viironen et al. (2009) (V09 from now on). Here wecarry out the search in the complete IPHAS photometric cata-logue, covering almost the whole Northern Plane including alsonon-stellar objects and objects without detection in the i ′ band.We aim at picking up all the possible PNe included in the photo-metric catalogue but due to the source detection technique usedin the catalogue generation (Gonz´alez-Solares et al. 2008), weare more likely to discover compact PNe. In addition to the over-all aim of the IPHAS PN project, gaining better understandingof the total Galactic PN population, we are especially interestedin the compact PNe as they are probably either very young ordistant objects. Thanks to the good spatial resolution of IPHAS,we are able to resolve subarcsecond objects which are ideal can-didates of young or proto-PNe. These, in turn, give us valuableinformation about the poorly known late stages of stellar evolu-tion of the PN progenitor stars.The discovery of even a few new very young PNe (vyPNe)would be important as only a few of them are known and notmany are expected in the Galaxy. For example, assuming thatthe PN lifetime is 20000 years and the estimated Galactic popu-lation 30000, we can expect that the Galactic population of PNeyounger than 1000 years is 1500. Of the expected PNe, only ∼ ∼
10 new vyPNe in the IPHAS area. But as our search is muchdeeper and has better spatial resolution than previous searches,we expect to find a larger number of vyPNe.
2. Search method
The source of the semi-automated PN search is the IPHASobject catalogue, including all the IPHAS observations untilJanuary 2008 (inclusive), thus covering 93% of the total IPHASarea. The initial data release has been published and a detaileddescription of the data reduction and products is provided inGonz´alez-Solares et al. (2008).Our PN candidate search consists of cleaning the cataloguedata from possible false detections and of choosing the PN can-didates due to their location in the IPHAS and Two Micron AllSky Survey (2MASS, Skrutskie et al. 2006) colour-colour dia-grams.
The IPHAS catalogue includes about 600 million detectionsof Northern plane objects, most of them detected two or moretimes. We require detections at least in the H α and r ′ filters. We can assume that a PN is visible in the continuum r ′ filter due topresence of the H α line while a detection in the i ′ filter is notassured. We also require a matching distance between the detec-tions in di ff erent filters of < . ′′ for nebular objects and < ′′ for stellar objects. These limits were defined empirically fromIPHAS detections of known PNe. Based on the known IPHAScolours of PNe (see V09) we start our candidate selection bymaking a generous colour cut: r ′ − H α > . r ′ − i ′ ) + . α in order not to eliminate possiblevery bright H α emitters. After these initial selection steps, wereduced the number of possible candidates to ∼
14 million.Most of the possible candidates after the initial selectionclearly have to be false detections. Studying the initial selection,we found the following problems: in cloudy nights, thicker cloudcover in the r ′ than in the H α image can cause apparent H α emit-ters; areas of di ff use H α emission contain detections of arbitrarynebular objects; in some cases a satellite crossing the H α -imageis matched with a star at the same position in r ′ and i ′ causing itto be detected as an H α emitter. To remove these kinds of prob-lems, we require that the objects must be detected at least twiceand at least one of these two observations must be in a field thatfulfils the IPHAS quality criteria, in line with the PhotoObjBestrequirements in Gonz´alez-Solares et al. (2008). About 99% ofthe IPHAS objects are observed at least twice, which allows usto apply this selection. For bad quality fields, their observationwill be repeated later so that the objects possibly lost in this stepwill eventually be recovered. After these selection steps we areleft with ∼ . i ′ -measurement in some of the observations but notin others, only the good quality images with i ′ detection are av-eraged. We also make a cut in H α magnitude, m(H α ) <
19. Ourmotivation for this magnitude cut is that the limiting magnitudeof IPHAS is m( r ′ ) ≃ r ′ - H α colour of a pure H α emit-ter (assuming that all the flux in the r ′ filter comes from the H α line) is 3.1 (Drew et al. 2005). Therefore to avoid being biasedtowards objects with smaller H α excess at faint magnitudes, themagnitude cut is a reasonable choice. This also reduces problemscaused by rising photometric errors at fainter magnitudes. Afterthe averaging and the magnitude cut, we are left with ∼ α because of the slightly broader PSF in this filter due to thelonger exposure time. Hence the binary is measured as an ex-tended object in the H α image while it is resolved in r ′ and i ′ .The two stars in r ′ and i ′ are then matched with the same ’nebu-lar’ H α object. We removed these false detections by eliminatingall objects that have two r ′ detections within 2 ′′ for the same H α detection. The radius of 2 ′′ was defined empirically. After re-moving this kind of false emitter, we are left with 88000 objects. . Viironen et al.: Candidate planetary nebulae in the IPHAS photometric catalogue 3 -2 -1 0 1 2 3 4 r’ - i’ r ’ - H α E(B-V) = 0 zone 1
E(B-V) = 4 zone 2A V = 3 mag H - K s J - H E(B-V)=0 E(B-V)=4A V = 3 mag Fig. 1.
Two-colour diagrams –
Left
IPHAS colours,
Right + ff erent reddenings are shown as solid lines and the colour cuts appliedin the candidate selection as dashed lines (see text).For these objects, the next selection step is based on their IPHASand 2MASS colours. After having cleaned the candidates using the above steps, near-infrared counterparts of the remaining IPHAS objects weresought. A matching radius of 1.4 ′′ was used when search-ing for matches in the 2MASS Extended Source Catalogue,while for the Point Source Catalogue a 1 ′′ search radius wasused. Note that the external astrometric precision of IPHASwith respect to 2MASS is generally better than 0.1 arcsec(Gonz´alez-Solares et al. 2008). If the object has both an ex-tended and a point source counterpart, the latter was selected.In the case that no 2MASS counterpart was found, UKIDSS data was used in the colour selection instead. Having IPHASand 2MASS / UKIDSS information in hand, the PN candidateswere selected based on their locations in the IPHAS ( r ′ - H α ) vs.( r ′ - i ′ ) and 2MASS ( J − H ) vs. ( H − K S ) diagrams. No colourcorrection was applied to UKIDSS data as the corrections rela-tive to the 2MASS photometric system are small (Hodgkin et al.2009). The colour selection is based on the location of knownPNe and other classes of emission line objects and normal starsin these diagrams. The diagrams were discussed in Corradi et al.(2008) and V09 and the candidate selection method is the samehere as in V09. The zone 1 and 2 (see Fig. 1) introduced in V09are defined by the following equations a) and b), respectively:a) r ′ − H α > . r ′ − i ′ ) + . r ′ − H α > . i ′ magnitude)b) 0 . r ′ − i ′ ) + . < r ′ − H α < . r ′ − i ′ ) + . . < r ′ − H α < . i ′ magnitude) The UKIDSS project is defined in Lawrence et al. (2007). UKIDSSuses the UKIRT Wide Field Camera (WFCAM, Casali et al. 2007) anda photometric system described in Hewett et al. (2006). The pipelineprocessing and science archive are described in Irwin et al. (2009) andHambly et al. (2008). We used data from the 4th data release. and J − H < . H − K s ) − . . The objects that fulfil the IPHAS criterion a) and the 2MASScriterion b) have the most probable colours to be genuine PNe.To be more complete in these highest probability zones, we alsoselected the objects having only a single IPHAS detection forvisual inspection if they fulfil these colour criteria.After the colour selection we are left with 4740 potential PNcandidates.
As a final selection step, all 4740 objects were checked visu-ally in their IPHAS preview images. With a few exceptions, theremaining visually distinguishable mimics are detections in ar-eas of large H α emitting nebulae such as H ii regions or super-nova remnants. In some cases, a faint star projected on an ex-tended nebulosity can cause a false H α emitter detection becauseof poor subtraction of the highly variable background. If this isclearly the case, these objects are removed, otherwise the objectwas retained. In general, if the non-PN nature was clear from themorphology, the object was discarded from the candidate list inthis visual study. If not, the object was included in the candidatelist and the final decision on its nature left for future spectro-scopic studies. Very extended objects are discarded, as the mor-phology is not well seen in the preview images. These objectswill be recovered in the extended nebula search (S09) of IPHASimage mosaics. Generally this size limit is ∼ ′ but a few moreextended nebulae with bright cores are included in our candidatelist. The final result of our PN search is a list of 1005 objects. Theseare listed in Table 1 which is available only electronically. Wesearched first for possible SIMBAD entries for our candidatesand found 311 matches. Of these, 168 have a primary object typeof PN or possible PN. Other relatively frequent objects worthmentioning are 34 emission objects or emission line stars, of
K. Viironen et al.: Candidate planetary nebulae in the IPHAS photometric catalogue r’ i’H α Fig. 2. H α , r ′ and i ′ IPHAS preview images, from left to right, ofa few PN candidates. The image sizes are 1 ′ × ′ , north is up andeast towards left. The IPHAS detections are shown as circles.The three images in the bottom row illustrate the selection of acondensation in a new extended PN candidate.which three have as a secondary object type PN and accordingto the literature two of these are indeed PNe, 18 H ii regions, 19Herbig-Haro (HH) objects, 13 nebulae of unknown nature, 11galaxies and 11 young stellar objects (YSOs), one being classi-fied as candidate only. Our selection also picked out six knownnovae. We have added the SIMBAD information to our final can-didate table. We also checked all references in the literature forthe 311 objects classified in SIMBAD. In 224 cases we consid-ered the object classification well established, and we did notstudy these objects any further.The 87 objects for which we considered that the object natureis not well known are further studied together with the 694 can-didates without any SIMBAD classification. Thus, we are leftwith a total of 781 PN candidates. Of the 83 candidates fromthe list of Witham et al. (2008) which were already published inV09 (as marked in Table 1), all except 10 objects are selectedhere as well. The missing objects were not selected because theWitham et al. (2008) selection was made before a recent changein the H α filter calibration had been implemented and the objectsnow fall outside our colour cuts or in the fields not fulfilling theIPHAS quality criteria.In addition to compact candidates, we also found many ex-tended objects due to catalogued condensations in larger neb-ulae. In these cases, in the final candidate list, the brightest(in H α ) catalogued condensation and the corresponding mag-nitudes are listed. In addition, the coordinates of the centre and -0.5 0 0.5 1 1.5 2 2.5 3log(H α /[SII])-1-0.500.511.52 l og ( H ( α / [ N II]) (cid:0)(cid:0)(cid:1)(cid:1) (cid:0)(cid:0)(cid:1)(cid:1) (cid:0)(cid:0)(cid:1)(cid:1)
HHSNR HII PNe
Fig. 3.
The S2N2 diagram of Sabbadin et al. (1977) showing thezones for SNRs and H ii regions, the 0.85 probability ellipse fornormal PNe (Riesgo & L´opez 2006), and the HH zone (V09).Over-plotted is our sample of 27 spectroscopically observed PNcandidates (diamonds). Three new IPHAS vyPNe studied in V09are shown as filled diamonds.the largest extension of the nebula are given. A detailed studyof the extended nebulae in IPHAS will be published later (S09).Examples of new IPHAS PN candidates are shown in Fig. 2.
3. Completeness and reliability of the catalogue
The catalogue we base our study on covers 93% of the IPHASarea. The PN search in the remaining 7% will be carried outwhen the data are available. Our aim is to discover as many newGalactic PNe as possible. However, as the search presented hereis carried out in the IPHAS photometric catalogue, we can aimto be complete only for compact PNe ( ≤ ′′ ). Of these objects,we expect to lose 4% in our candidate selection process: 1% dueto the requirement of a minimum of two detections and 3% dueto the applied colour selection.We tested this in practice by checking if the 63 knowncompact ( ≤ ′′ ) PNe (Acker et al. 1994; Parker et al. 2006;Miszalski et al. 2008) in the IPHAS area and inside the IPHASbrightness limits were selected by our algorithm. Six of thesewere not picked up: 4 because of problems in the IPHAS cata-logue generation which sometimes splits a single extended ob-ject into several objects, and 2 due to our selection algorithm, inline with our prediction above.We have carried out our search aiming to be as completeas possible rather than to have a clean selection of PNe. Ofthe possible mimics, we expect that the most common are T-Tauri stars because they enter our colour selection (see V09 andSection 4.2.1) and they can be expected to be numerous objects.Possible mimics are also symbiotic stars because these objectsenter clearly into our colour selection (see V09) and their pre-dicted number is large (from 3 × to 4 × Galactic systems,see Corradi et al. 2008, and references therein). Further potentialmimics are HH objects linked with young stars, although theseare discarded in the visual inspection if the morphology allowsa clear separation from PNe. Also, massive young stellar ob-jects (mYSOs) and H ii regions are possible mimics. The latterwere again usually discarded in the visual inspection, howevercompact H ii regions especially might have remained in the finalcandidate list. . Viironen et al.: Candidate planetary nebulae in the IPHAS photometric catalogue 5
4. The nature of the candidates
In order to confirm the PN nature of our candidates, opticalspectra (as a minimum) are needed. For this purpose we havestarted a follow-up spectroscopy campaign. So far we have spec-tra for 69 of the PN candidates using the 2.1m telescope atthe Observatorio Nacional de San Pedro M´artir (SPM), Mexico,Isaac Newton Telescope (INT), William Herschel Telescope(WHT), and Nordic Optical Telescope (NOT), La Palma, Spain,and the Australian National University 2.3-m telescope at SidingSpring Observatory (SSO), Australia. These include the 19 spec-tra mentioned in V09. In this paper, the results from the spectro-scopic study are only used to assess the e ffi ciency of our PN se-lection. Detailed study of the individual spectra will be publishedin subsequent papers. A few of these objects have been alreadypublished by Parker et al. (2006) or Miszalski et al. (2008) (asindicated in Table 1). However, as they were independently dis-covered from IPHAS, they will be included here in our follow-upspectroscopy statistics. Of the 69 observed objects, 38 show a spectrum with nebularlines typical of normal PNe. We refer to these as IPHAS nebulaein the following discussion. Closer inspection of the 38 spectraled us to classify 4 of them as likely symbiotic stars (for de-tails about recognising symbiotic star spectra, see Corradi et al.2008). We refer to these as IPHAS symbiotics.To analyse the nature of the remaining objects we placedthem into the log(H α / [N ii ]) vs. log(H α / [S ii ]) (S2N2) diagnos-tic diagram (as discussed by V09), which is a revised versionof the original S2N2 diagram by Sabbadin et al. (1977). 27 ofthe remaining 34 objects show all the necessary emission linesin their spectrum, see Fig. 3. Of these, 16 are located within thearea of the 0.85 PN probability ellipse and outside the overlap-ping areas of other types of objects. We will refer to these 16objects as IPHAS PNe.It is important to emphasise that objects not included withinthe 85% probability ellipse but showing typical PNe lines mayalso very well be genuine PNe. The same can be true for the 7IPHAS nebulae not plotted in Fig. 3. As a matter of fact, we havemarked in Fig. 3 three new PNe discussed in V09 and they all arelocated outside the PN ellipse (one of the V09 new young PNedoes not show [S ii ] lines in its spectrum and is thus not plottedhere). The objects outside the ellipse are probably peculiar anda detailed study is needed in order to classify them.The 31 clear non-PNe belong to other classes of H α emittingobjects such as YSOs. They are interesting newly discovered ob-jects on their own and a study of their spectra is in progress(Valentini et al. 2009) but in our search for new PNe they areset aside as mimics. We will refer to them as IPHAS emissionline stars, representing the sample of undesired objects in ourcandidate selection. The sample of objects with follow-up spectroscopy is small forrobust statistics but we estimate that about 16% of the 781 can-didates are genuine, normal PNe. We derived this result with thefollowing reasoning. 28% of the candidates are located in zone 1in the IPHAS two-colour diagram. There are 16 IPHAS PNe, 10 IPHAS emission line stars and 3 IPHAS likely symbiotic starsin zone 1. From these data we can deduce that 55% of the zone 1objects are PNe. On the other hand, from the location of knownPNe in the IPHAS two-colour diagram (see V09), we know thatthe zone 1 objects represent 95% of the PN population – the restbeing in zone 2. This leads to the quoted total PN match rate of0 . × . / . = . ii ] dou-blet in emission in our spectrum.However, we note that 90% of our candidates are of diameter ≤ ′′ . Even a PN match rate of only 16% means that our can-didate list includes 112 compact PNe in the Northern Galacticplane. This would roughly triple the number of known compactPNe in the IPHAS area. Considering that we expect to discover aso far hidden population of vyPNe, like the four objects in V09,we assume this number to be even higher. In addition to the IPHAS and 2MASS two-colour diagrams, westudied the location of our candidates in various diagnostic di-agrams combining IRAS (The Infrared Astronomical Satellite,Beichman et al. 1988), 2MASS and MSX (Midcourse SpaceExperiment, Egan et al. 1999) infrared data, in cases where theywere detected.In all of the diagrams we have only plotted those PN can-didates which have the needed fluxes well measured. We haveseparately indicated the locations of known PNe included in ourcandidate list. We have also marked the locations of the IPHASPNe, IPHAS emission line stars, and IPHAS symbiotic stars. Asthe sample of new IPHAS objects (for which we have follow-up spectroscopy) is small, we have plotted them even when onlya limit of the flux is available. In these cases, arrows indicatethe direction for the possible location of the source in the dia-gram. As we expect our major mimics to be T-Tauri stars andsymbiotic stars, we have studied the location of these objects inthose diagrams where this work has not previously been done byother authors. For this purpose we have searched the data for thesymbiotic star sample of Belczy´nski et al. (2000) and the T-Tauristars of Herbig & Bell (1988).As mentioned above, in addition to normal PNe, we expectthat our candidate list includes vyPNe or even proto-PNe, likethe four objects presented in V09. To get an idea about the ex-pected locations of these kinds of objects in the diagnostic dia-grams, we have also plotted in them the ’very-likely post-AGBstars’ of Szczerba et al. (2007).
K. Viironen et al.: Candidate planetary nebulae in the IPHAS photometric catalogue -2 -1 0 1 2 3 4 r’ - i’ r ’ - H α E(B-V) = 0 zone 1
E(B-V) = 4 zone 2A V = 3 mag H - K s J - H A V = 3 magE(B-V)=0 E(B-V)=4 Fig. 4.
Left
Location of the new IPHAS PNe (filled diamonds), new IPHAS emission line stars (filled circles), and new IPHASsymbiotic stars (filled triangles) in the IPHAS two-colour diagram. Plotted are also the locations of Herbig & Bell (1988) T-Tauristars (squares) and the post-AGB stars of Szczerba et al. (2007) (plus signs). The known PNe in our candidate list are shown asasterisks. Main sequence tracks are shown as solid lines and the colour cuts applied in the candidate selection as dashed lines (seetext).
Right
The IPHAS and 2MASS two-colour diagrams for the objects forwhich we have spectra are shown in Fig. 4. The error bars are notplotted but are given in Table 1. However, generally the errorsare about the same size as the symbols used. The locations ofthe newly discovered objects are in line with the locations ofthe previously known objects (see V09). The location of T-Tauristars in IPHAS was not shown in V09 and here we see that someT-Tauri stars enter our zone 2 in the IPHAS diagram.Only 42 post-AGB stars from Szczerba et al. (2007) have anIPHAS counterpart. These objects do not necessarily yet showH α in emission which is apparent in the IPHAS two-colour dia-gram. Below our selection line in 2MASS, the post-AGB objectsare located towards higher J − H and H − K s colours as comparedto PNe. We note that the new young V09 PNe also generally havelower r ′ − H α colours and higher J − H and H − K s colours thanthe bulk of the normal PNe.To conclude, we consider a higher probability for our candi-dates to be normal PNe if they are located high up in the IPHAStwo-colour diagram and towards low J − H and intermediate H − K s colours in the 2MASS two-colour diagram. However,for vyPNe we expect a lower position in the IPHAS two-colourdiagram and larger J − H and H − K s colour values. We cross-matched our candidate list with the IRAS catalogue,searching for possible matches in the IRAS Point SourceCatalog, IRAS Faint Source Catalog, IRAS Cataloged Galaxiesand Quasars, IRAS Serendipitous Survey Catalog and in theIRAS Small Scale Structure Catalog. When searching for IRASdata, a 16 ′′ search radius was used. This choice is based on thepositional accuracy of extended IRAS objects (Beichman et al.1988). Of our 781 PN candidates, 51 are well measured (at least)in the 12 µ m, 25 µ m and 60 µ m IRAS bands.Pottasch et al. (1988) defined di ff erent areas marking thelocation of H ii regions, OH / IR stars, PNe, normal late -0.5 0 0.5 1 1.5 2log(F25/F12)-1-0.500.511.5 l og ( F / F ) PNOH/IRT-Tau OutflowHIIOH-Mira GalaxyStar
Fig. 5.
IRAS two-colour diagram. The symbols used are thesame as in Fig. 4. Shown are also the locations of our candi-dates (open diamonds) and the Belczy´nski et al. (2000) symbi-otic stars with an IRAS couterpart (open triangles).type stars, and galaxies in the IRAS F (12 µ m) / F (25 µ m) vs. F (25 µ m) / F (60 µ m) diagram, as shown in Fig. 5. We also addedto the diagram the locations of T-Tauri stars as studied byEmerson (1987) and of outflow sources (both young and evolvedstars with fast bipolar outflows) from Zijlstra et al. (2001).The locations of the newly discovered objects are again inline with expectations except for one IPHAS PN which is clearlyabove the PN area, in the Galaxy / H ii region zone. However, thelocation of this object, IPHASXJ195248.8 + . Viironen et al.: Candidate planetary nebulae in the IPHAS photometric catalogue 7 We also searched for MSX counterparts for our PN candidates.A 5 ′′ search radius was used based on the MSX positional accu-racy (Egan et al. 1999). 122 of our PN candidates have an MSXcounterpart well measured at least in one of the MSX filters.Lumsden et al. (2002) studied various diagnostic diagramscombining MSX and 2MASS flux ratios in order to select mYSOcandidates. For this purpose, they placed Herbig Ae / Be stars,mYSOs, methanol maser sources with radio emission (whichtrace massive star forming regions), compact H ii regions, carbonstars, OH / IR stars, and PNe into the diagrams. We refer here tothe diagrams shown in Figures 3, 4, 9 and 10 in Lumsden et al.(2002) as L1, L2, L3 and L4, respectively. In each of these dia-grams we defined a line along the reddening vector which lim-its the zone where most of the known PNe in the diagrams arelocated. These cut lines in the L1, L2, L3 and L4 diagrams, re-spectively, are as follows:log[ F (14 µ m ) / F (8 µ m )] > − .
64 log[ F (12 µ m ) / F (14 µ m )] + F (14 µ m ) / F (12 µ m )] < .
56 log[ F (21 µ m ) / F (8 µ m )] − . F (8 µ m ) / F ( K s )] < .
03 log[ F (12 µ m ) / F (8 µ m )] − . F ( K s ) / F ( J )] < .
29 log[ F (21 µ m ) / F (12 µ m )] − . . We studied the locations of our candidates, symbiotic stars,T-Tauri and post-AGB stars in the diagrams L1, L2, L3 and L4and found that all of them separate PNe from T-Tauri and sym-biotic stars quite nicely. However, we note that in all these dia-grams, the PN zone overlaps with the zones occupied by com-pact H ii regions and methanol maser sources with radio emis-sion. There is also a partial overlap with the OH / IR star zone.In the L3 and L4 diagrams, also some Herbig Ae / Be stars andmYSOs enter the PN zone. Nevertheless, as we expect T-Tauriand symbiotic stars to be the most frequent mimics in our cata-logue, we consider these diagrams useful in weighting the prob-ability that our candidates are a genuine PNe. However, the post-AGB stars are not separated from the rest of the objects in any ofthe diagrams. This can mean that the vyPNe would not be either.
We have studied in various diagnostic diagrams the locations ofour candidates and the expected most frequent mimics, namelyT-Tauri stars and symbiotic stars. The Lumsden et al. (2002) di-agrams do not separate PNe from compact H ii regions but theseparation from other possible mimics is generally good, espe-cially in the IRAS diagram and in the L1 and L2 diagrams. TheIR data certainly can be used to further restrict the possibilitythat our candidates are genuine PNe in the cases where the nec-essary IRAS, MSX and 2MASS data for the candidate exist.We have also placed post-AGB stars in these diagrams aswe expect that our candidate list includes, in addition to normalPNe, vyPNe or proto-PNe like those presented in V09. Basedon the locations of the post-AGB stars as compared to PNe weexpect that the vyPNe are found outside the most typical PNzones in IPHAS and 2MASS two-colour diagrams. The resultsin V09 support this observation. In these zones the confusionwith mimics is higher and thus makes the discovery of vyPNemore di ffi cult. In the IRAS and MSX diagrams the locationsof post-AGB stars overlap with both PNe and normal late typestars. This might be an evolutionary trend, the most evolved post-AGB stars being located in the PN zone while the less evolved ones are located in the zone of late type stars. We find that wecannot separate the vyPNe from the normal late type stars basedonly on their infrared colours. Lastly, we note that in the IRASdiagram, the distribution of our candidates clearly di ff ers fromthe distribution of known PNe and post-AGB stars. Therefore,we conclude that most of our candidates with an IRAS coun-terpart are rather mimics than young PNe. We have included allavailable IR data in Table 1.
5. Additional data
Candidates with PN-like colours in the di ff erent optical / IR di-agrams, showing an adequate morphology and associated withradio continuum sources, are almost certainly PNe. We searchedfor matches to our candidates by comparing the IPHAS andNVSS (NRAO VLA Sky Survey, Condon et al. 1998) radio im-ages. 63 of the 781 IPHAS PN candidates were found to have an1.4 GHz radio counterpart. We add the radio information to ourcandidate table.
Normal PNe are expected to be more extended than T-Tauri stars,symbiotic stars, post-AGB stars and other stars and we mea-sured the diameters of all our candidates. For the 75 clearly ex-tended candidates, the diameters and coordinates of the centrewere measured in the IPHAS H α images using the FITS viewerds9 . For the 706 candidates that appear compact, we have mea-sured their full width half maximums (FWHMs). For compari-son, we measured the FWHMs of field stars in an area of 10 ′ surrounding the objects, selecting the stars within ± . σ clipped mean of these defines the stellar FWHM and the RMSits error, as well as the error in the FWHM measurement ofthe object. We decided to use both trimming and sigma clip-ping for a robust measurement of the stellar FWHM. In orderto claim that one candidate is resolved, we use a conservativelimit of five sigma, i.e. we consider that the object is resolved if FWHM ( ob ject ) − FWHM ( stars ) > × err FWHM ( ob ject ).Of the 706 compact objects, 124 turned out to be resolved us-ing this criterion. Of these, 25 have a deconvolved diameter, p FWHM ( ob ject ) − FWHM ( stars ) , smaller than 1 arcsec.120 of the resolved candidates are of size ≤ ′′ , the size rangefor which our search method is optimised. At diameters greaterthan 5 ′′ , only a fraction of the nebulae where the semi-automatedalgorithm detected a small condensation is found. We hardly findany candidates with diameters smaller than 0 . ′′ , as expected forinstrumental and observational reasons. There are 75 candidateswith diameters in the range 0 . ′′ − ′′ . This is the diameter rangewhere young / compact / very distant PNe are expected, possiblyaccompanied by a small fraction of nebulae around T-Tauri starsand symbiotic stars. From 2 ′′ on, the number of objects clearlydrops with increasing diameter. This can be attributed to the dif-ficulty in detecting lower surface brightness objects at large dis-tances due to interstellar extinction. http: // / RD / ds9 K. Viironen et al.: Candidate planetary nebulae in the IPHAS photometric catalogue Fig. 6.
IPHAS H α images of the 14 extended candidates with PN-like morphology and which have an entry in SIMBAD but whosenature is not confirmed.
6. Extended candidates
Although our search method is optimised to discover compact( < ′′ ) PNe, we also found 81 extended objects with a diame-ter > ′′ . For these clearly extended objects their morphologycan help in determining their nature. 36 nebulae show regularmorphology so that from their appearance alone we can assumea higher probability that these objects are genuine PNe. Twoof these were discovered by Miszalski et al. (2008). We presenthere the images and short descriptions of the remaining 34 ob-jects. Of these, 14 are known in SIMBAD but their classificationis not well established and they deserve further study. IPHASXJ022045.2 + Classified as a galaxy (Hau et al.1995). However, it is hardly visible in the IPHAS continuumfilters, which is an argument against the galaxy classification.Possibly an elliptical PN.
IPHASXJ052708.2 + Classified as a reflection nebula(Magakian 2003) but no detailed study of the object is available.Possibly a round PN.
IPHASXJ055242.8 + A CO source (Wouterloot et al.1993). Possibly a round PN.
IPHASXJ190438.7 + Catalogued as an H α -emissionstar (Kohoutek & Wehmeyer 1999). Condon et al. (1999) haveobtained an optical spectrum of this object and classify it asa PN. Preliminarily classified as a symbiotic star based on aMASH spectrum but more observations are needed. An ellipticalPN or symbiotic star. IPHASXJ190954.7 + Classified as a possible PN(Kronberger et al. 2006) but no spectroscopic confirmation isavailable. Possibly a round or bipolar PN with a brightenedwaist.
IPHASXJ192751.3 + Classified as a possible PN(Preite-Martinez 1988) but no spectroscopic confirmation isavailable. Possibly a round PN.
IPHASXJ193141.3 + Classifications of PN(Condon et al. 1999), Galaxy (Roman et al. 2000) and possiblePN Preite-Martinez (1988) but no spectroscopic confirmationis available. Hardly visible in the IPHAS continuum filters andthus unlikely to be a galaxy. Possibly a round / slightly ellipticalPN. . Viironen et al.: Candidate planetary nebulae in the IPHAS photometric catalogue 9 Fig. 7.
IPHAS H α images of the 20 extended candidates with PN-like morphology. IPHASXJ194040.3 + Classified as a possible PN byRoman et al. (2000) and as a PN by Kronberger et al. (2006) butno spectroscopic confirmation is available and hardly visible inthe IPHAS continuum filters. Possibly a round PN.
IPHASXJ194226.1 + Classified as a possible PN(Kronberger et al. 2006) but no spectroscopic confirmation isavailable. Possibly a bipolar / quadrupolar PN. IPHASXJ195209.1 + Classified as PN (PN K 3-48).Many studies refer to this object as a PN but is listed as an H ii region by Acker et al. (1994). No spectra available in the litera-ture. Possibly an elliptical PN. IPHASXJ200018.7 + Classified as a possible PN(Acker et al. 1994) but with no spectroscopic confirmation.Possibly a round PN.
IPHASXJ200457.3 + Listed as a ’Bright Nebula’(Kronberger et al. 2006). Possibly a round or a bipolar PN withbrightened waist.
IPHASXJ202549.3 + Catalogued as an H α emission linestar (Kohoutek & Wehmeyer 1997), PN (Manchado et al. 1989)and H ii region (Garc´ıa-Lario et al. 1997) but with no spectro-scopic confirmation available. Possibly a bipolar PN. IPHASXJ232713.1 + Classified as possible PN(Kronberger et al. 2006) but with no spectroscopic confirmationavailable. Possibly a round PN.
IPHASXJ002059.6 + A bipolar nebula.
IPHASXJ025658.2 + Possibly bipolar but a deeper im-age is needed to define the morphology.
IPHASXJ185309.4 + A roundish / slightly elliptical neb-ula. IPHASXJ191345.6 + A round nebula. Brighter rims, es-pecially the western one.
IPHASXJ191445.1 + Round nebula. Brightened rim.
IPHASXJ191727.0 + Faint nebula. Looks round but adeeper image is needed.
IPHASXJ192140.4 + Butterfly shaped nebula.Preliminarily classified as a symbiotic star based on aMASH spectrum but more spectra are needed to confirm theclassification.
IPHASXJ192902.5 + A round nebula.
IPHASXJ193532.1 + A round nebula.
IPHASXJ193718.8 + A bipolar nebula. Only two irreg-ularly shaped lobes visible.
IPHASXJ193949.8 + Shape unclear but if the brightcentral part is the waist of a bipolar nebula, the real extent canbe larger than the 10 ′′ listed in Table 1. IPHASXJ194648.2 + A bipolar nebula. The two lobesare combined with an elliptical ring.
IPHASXJ194940.9 + A bipolar nebula. The waist isclearly brighter than the lobes.
IPHASXJ195126.5 + Faint round / elliptical nebula. IPHASXJ195248.8 + Bipolar nebula. The waist is brightwhile the lobes are very faint.
IPHASXJ204118.3 + The morphology is not very clearbut looks like a bright waist of a bipolar nebula with faint lobesonly partially visible.
IPHASXJ212000.1 + An elliptical nebula. The NW andSE rims are brightened.
IPHASXJ212335.3 + A bipolar nebula, bright waist andonly partly visible lobes.
IPHASXJ212608.4 + An ’S’-shaped nebula.Quadrupolar? Deeper imaging needed to define the mor-phology.
IPHASXJ214032.5 + A round nebula.
7. The IPHAS PN candidate list
We have collected all the information discussed above into a listof IPHAS PNe candidates (provided in electronic format only).The table contains all the 1005 objects picked up by our selectionalgorithm. The columns of the table are as follows:
Name : The IAU-registered sexagesimal, equatorial position-based source name in the form: IPHASJhhmmss.ss + ddmmss.sfor stellar and IPHASXJhhmmss.s + ddmmss for extended ob-jects. ”J” indicates the position is J2000. RA, DEC : The right ascension and declination (J2000) of theobject catalogued by IPHAS [deg].
Gal : The galactic coordinates of the object in the form lll.l + bb.bwhere l and b stand for galactic longitude and latitude, respec-tively, truncated to one decimal place [deg]. r, r-i, r-Ha : The IPHAS r ′ band magnitude, r ′ − i ′ (if measuredin i ′ ) and r ′ − H α colours of the object. In the case of extendedobjects, the IPHAS magnitude / colour information correspondsto the brightest catalogued H α condensation of the object. f r-Ha : IPHAS photometry flag, meaning that the object is satu-rated in H α . cRA, cDE : The right ascension and declination (J2000) of themeasured centre for the object [deg]. This information is onlypresented for extended objects. IRCat : The near-IR catalogue (2MASS PSC, 2MASS XSC, orUKIDSS) from which the near-IR data of the object was taken.
J, J-H, H-Ks : J band magnitude, J − H and H − K s colours ofthe object. IRASCat : The IRAS catalogue (PSC / SSC / SSS) from which theIRAS data of the object was taken. f12, f25, f60, f100 : IRAS fluxes at 12, 25, 60 and 100 µ m [Jy]. fMSXa, fMSXc, fMSXd, fMSXe : MSX fluxes at 8.28, 12.13,14.65 and 21.34 µ m [Jy]. fNVSS : Radio flux at 1.4 GHz [mJy]. sFWHM : The stellar FWHM [arcsec]. oFWHM : The object FWHM [arcsec]. The FWHM informa-tion is provided only for the objects appearing quasi-stellar inIPHAS. Diam : Diameter for the clearly extended candidates [arcsec].
PNIRAS : In the PN zone in the IRAS diagram [yes / no]. PNL1 : In the PN zone in the L1 diagram [yes / no]. PNL2 : In the PN zone in the L2 diagram [yes / no]. PNL3 : In the PN zone in the L3 diagram [yes / no]. PNL4 : In the PN zone in the L4 diagram [yes / no]. score : The PN score as defined below. f SIMBAD : Flag indicating that we consider the SIMBAD typenot well established. type : The types listed in SIMBAD. SIMBAD : The SIMBAD name. crossref : Cross reference with the other IPHAS and AAO-UKST H α Survey selected catalogues: C = IPHAS symbioticstar candidates (Corradi et al. 2008), S = IPHAS extended nebu-lae catalogue (S09), M = PN in MASH or MASH-II catalogues(Parker et al. 2006; Miszalski et al. 2008), W = IPHAS emittercatalogue (Witham et al. 2008). class : Preliminary classification based on follow-up spec-troscopy. A detailed analyses of the objects and the spectra willbe published elsewhere. Objects with a MASH-II classificationhave been followed up by the MASH project. (IPHAS = spec-trum taken but not yet studied, IPHAS PN / MASH-II PN = likelyplanetary nebula, IPHAS nebula = possible PN, IPHAS / MASH-II star = H α emission line star, IPHAS / MASH-II symbiotic = likely symbiotic star, See S09 = will be published in S09, SeeV09 = was studied in V09).In addition the magnitude and colour errors are given in thecolumns e x and the flux qualities in the columns q x for the xdata.The PN score is defined from the source location in theIPHAS, 2MASS and other IR diagnostic diagrams in the fol-lowing way: In the IPHAS two-colour diagram, the distance inmagnitudes to the lowest selection line is calculated. To this weadd the calculated distance in magnitudes from the selection linein the 2MASS two-colour diagram. If the object is located to theleft of this selection line the distance is simply set to zero. Ifin addition the object is located in the PN zone in any of theinfrared diagrams, its score is increased by one for each IR dia-grams where the object is in the right zone. So, in summary, the For the near-IR data an error value of zero indicates that the magni-tude, or one of the magnitudes defining the colour, is a 95% confidenceupper limit (Cutri et al. 2003). For IRAS PSC and SSC: 3 = high quality, 2 = moderate quality,1 = upper limit. For IRAS SSS: A = high quality, B = intermediate qual-ity, F = low quality (Beichman et al. 1988). For MSX: 4 = excellent, 3 = good, 2 = fair, 1 = limit (Egan et al. 1999).. Viironen et al.: Candidate planetary nebulae in the IPHAS photometric catalogue 11 higher the PN score of the object is, the higher we consider thepossibility that this candidate is really a PN. For example, forthe known PNe in the candidate list, the average PN score is 3.6while for the other types of objects classified in SIMBAD theaverage is 2.5. For the new IPHAS PNe the average score is 3.3and for the new IPHAS emission line stars 1.9.However, the PN score alone cannot obviously be used todiscard the possibility that a candidate is a PN. For example, thePN score of the known, extended PN, PN G036.9-02, is only0.52. Its H α excess is low due to an underlying star which waspicked up by our selection algorithm and it is not an IRAS norMSX source which explain the low score.In addition to the PN score, the radio and diameter data giveadditional constraints on the probable nature of the object.
8. Summary and discussion
IPHAS provides a powerful database for PN searches in theNorthern Galactic Plane. Thanks to its high resolution and depth,especially the two extremes of the PN evolution, young, compactPNe and old, faint, and extended PNe are explored. The visualsearch, optimal for the discovery of very extended PNe, is un-derway (S09) while here we have presented the semi-automatedsearch for PNe based on the IPHAS photometric catalogue. Thismethod is optimised to discover small-sized PNe but can alsodiscover some extended PNe. In addition to normal PNe, we ex-pect that our candidate list includes vyPNe and proto-PNe.We present a list of 1005 objects selected from the initialcatalogue of ∼
600 million IPHAS detections. The SIMBADdatabase and literature study reveals that 224 of these are ac-tually known objects, 149 being confirmed PNe. This leaves uswith 781 PN candidates. About 25% of the selected candidatesare slightly extended and the clearly extended ones ( > ′′ ) over-lap with the visual search by S09 for extended PNe. This ensuresthat the combined IPHAS PN search is as complete as possible.When selecting the PN candidates, we have aimed to be com-plete rather than having a clean selection of candidates. The firstresults from our follow-up spectroscopy reveal that 38 out of 69candidates observed show nebular lines, whereas the remainingcandidates are H α emission line stars, most probably YSOs. Inorder to help in the selection of good PNe candidates we havestudied the locations of the candidates in a number of IR diag-nostic diagrams. Further, we provide additional information likethe sizes of the objects and the available radio data for them.The probability that a candidate is a genuine, normal PN ishigher if it is located in the PN area in the IPHAS and IR dia-grams and if it is extended in H α . All this information is pro-vided in our candidate list and the colour-colour diagram loca-tions are used to define a PN score whose value is higher forcandidates with properties like the known, normal PNe.However we would like to point out that exploring the ob-jects not fulfilling the ”normal PN” criteria listed above can besurprisingly productive in discovering objects in rare PN phases,as is shown by the very young PNe discovered and discussed inV09. Acknowledgements.
K. V., A. M., and R. L. M. C. acknowledge funding fromthe Spanish AYA2007-66804 grant. In addition, K. V. acknowledges the grantfrom the Magnus Ehrnrooth foundation, Finland. M. R. and G. D.-I. acknowl-edge support from Mexican CONACYT project 50359-F. This paper makesuse of data obtained as part of IPHAS carried out at the INT as well asINT, WHT, NOT and SPM2.1m spectroscopic data. The INT and WHT areoperated by the Isaac Newton Group and NOT by NOTSA on the island ofLa Palma in the Spanish Observatorio del Roque de los Muchachos of theInstituto de Astrof´ısica de Canarias. SPM 2.1m is operated by UNAM at theOAN of San Pedro M´artir Observatory, Mexico. All IPHAS data are processed by the Cambridge Astronomical Survey Unit, at the Institute of Astronomyin Cambridge. We also acknowledge use of data products from the 2MASS,which is a joint project of the University of Massachusetts and the InfraredProcessing and Analysis Centre / California Institute of Technology (funded bythe USA’s National Aeronautics and Space Administration and National ScienceFoundation).This research made use of data products from the Midcourse SpaceExperiment. Processing of the data was funded by the Ballistic Missile DefenseOrganization with additional support from NASA O ffi ce of Space Science. Thisresearch has also made use of the NASA / IPAC Infrared Science Archive, whichis operated by the Jet Propulsion Laboratory, California Institute of Technology,under contract with the National Aeronautics and Space Administration.
References
Acker, A., Marcout, J., & Ochsenbein, F. 1996, First Supplement to the SECGPN(Observatoire de Strasbourg)Acker, A., Ochsenbein, F., Stenholm, B., et al. 1994, VizieR Online DataCatalog, 5084Beichman, C. A., Neugebauer, G., Habing, H. J., Clegg, P. E., & Chester,T. J., eds. 1988, Infrared astronomical satellite (IRAS) catalogs and atlases.Explanatory supplement, Vol. 1Belczy´nski, K., Mikołajewska, J., Munari, U., Ivison, R. J., & Friedjung, M.2000, A&AS, 146, 407Casali, M., Adamson, A., Alves de Oliveira, C., et al. 2007, A&A, 467, 777Condon, J. J., Cotton, W. D., Greisen, E. W., et al. 1998, AJ, 115, 1693Condon, J. J., Kaplan, D. L., & Terzian, Y. 1999, ApJS, 123, 219Corradi, R. L. M., Rodr´ıguez-Flores, E. R., Mampaso, A., et al. 2008, A&A,480, 409Corradi et al. 2009, in preparationCutri, R. M., Skrutskie, M. F., van Dyk, S., et al. 2003, 2MASSAll Sky Catalog of point sources. (The IRSA 2MASS All-Sky Point Source Catalog, NASA / IPAC Infrared ScienceArchive. http: // irsa.ipac.caltech.edu / applications / Gator / )Drew, J. E., Greimel, R., Irwin, M. J., et al. 2005, MNRAS, 362, 753Egan, M. P., Price, S. D., Moshir, M. M., Cohen, M., & Tedesco, E. 1999, NASASTI / Recon Technical Report N, 14854Emerson, J. P. 1987, in IAU Symposium, Vol. 115, Star Forming Regions, ed.M. Peimbert & J. Jugaku, 19–30Frew, D. J. & Parker, Q. A. 2006, in IAU Symposium, Vol. 234, PlanetaryNebulae in our Galaxy and Beyond, ed. M. J. Barlow & R. H. M´endez, 49–54Garc´ıa-Lario, P., Manchado, A., Pych, W., & Pottasch, S. R. 1997, A&AS, 126,479Gonz´alez-Solares, E. A., Walton, N. A., Greimel, R., et al. 2008, MNRAS, 707Hambly, N. C., Collins, R. S., Cross, N. J. G., et al. 2008, MNRAS, 384, 637Hau, G. K. T., Ferguson, H. C., Lahav, O., & Lynden-Bell, D. 1995, MNRAS,277, 125Herbig, G. H. & Bell, K. R. 1988, Catalog of emission line stars of the orionpopulation (Lick Observatory Bulletin, Santa Cruz: Lick Observatory)Hewett, P. C., Warren, S. J., Leggett, S. K., & Hodgkin, S. T. 2006, MNRAS,367, 454Hodgkin, S. T., Irwin, M. J., Hewett, P. C., & Warren, S. J. 2009, MNRAS, 211Irwin et al. 2009, in preparationKohoutek, L. & Wehmeyer, R. 1997, Catalogue of stars in the northern MilkyWay having H-alpha in emission, ed. L. Kohoutek & R. WehmeyerKohoutek, L. & Wehmeyer, R. 1999, A&AS, 134, 255Kronberger, M., Teutsch, P., Alessi, B., et al. 2006, A&A, 447, 921Lawrence, A., Warren, S. J., Almaini, O., et al. 2007, MNRAS, 379, 1599Lumsden, S. L., Hoare, M. G., Oudmaijer, R. D., & Richards, D. 2002, MNRAS,336, 621Magakian, T. Y. 2003, A&A, 399, 141Manchado, A., Garc´ıa-Lario, P., Esteban, C., Mampaso, A., & Pottasch, S. R.1989, A&A, 214, 139Miszalski, B., Parker, Q. A., Acker, A., et al. 2008, MNRAS, 384, 525Moe, M. & De Marco, O. 2006, ApJ, 650, 916Parker, Q. A., Acker, A., Frew, D. J., et al. 2006, MNRAS, 373, 79Parker, Q. A., Phillipps, S., Pierce, M. J., et al. 2005, MNRAS, 362, 689Pottasch, S. R., Olling, R., Bignell, C., & Zijlstra, A. A. 1988, A&A, 205, 248Preite-Martinez, A. 1988, A&AS, 76, 317Riesgo, H. & L´opez, J. A. 2006, Rev. Mexicana Astron. Astrof., 42, 47Roman, A. T., Iwata, I., & Sait¯o, M. 2000, ApJS, 127, 27Sabbadin, F., Minello, S., & Bianchini, A. 1977, A&A, 60, 147Sabin et al. 2009, in preparation (S09)Skrutskie, M. F., Cutri, R. M., Stiening, R., et al. 2006, AJ, 131, 1163Szczerba, R., Si´odmiak, N., Stasi´nska, G., & Borkowski, J. 2007, A&A, 469,799Valentini et al. 2009, in preparationViironen, K., Mampaso, A., Corradi, R. L. M., et al. 2009, arXiv0904.1937 (V09)2 K. Viironen et al.: Candidate planetary nebulae in the IPHAS photometric catalogue